Desktop computers are generally ugly creatures. Unless you've got a shiny new Apple iMac, it's likely that you prefer to keep your case out of sight. However, with a few quid and a sprinkling of know-how, you can transform your old PC into a snazzy cube, mostly using parts you already have.

You might also have an old PC that's good only for spare parts - this could be the PC project you've been waiting to try your skills on. Or, of course, you might want to build a brand-new PC from scratch.

As well as looking the part, cube PCs are about a third of the size of desktop computers. Despite this, they contain most of the hardware you'd find in a standard desktop computer and will perform just as well in Windows applications and 3D games.

We've tried to keep costs to a minimum and use what we've already got to build our new cube. It's likely that you'll already own a monitor, mouse and keyboard but if you fancy upgrading other elements, such as the processor, memory or hard disk, you can buy new components from numerous online or high-street shops.

The case
The first thing to bear in mind when building a cube PC is the kind of case - or chassis - you're going to use. To keep things simple, we'd recommend opting for a barebones unit, which contains a motherboard with integrated graphics (so no need to fit a graphics card), on-board 5.1 sound and networking ports. This means all you need to add is a processor, hard disk, memory, an optical drive and an operating system.

While cube cases cost from £150, we plumped for AOpen's EX65 (£215 from Overclockers UK), which supported our existing 2GHz Pentium 4 processor, has an AGP slot in case you want to add a good graphics card in future, a spare PCI slot for adding a TV tuner card, space for an optical drive and a 3.5in drive bay.

These options may seem unnecessary now, but they give you room to expand the PC's abilities in future, meaning you won't have to buy or build a new PC when applications demand more power a few years down the line.

It's also laden with a great selection of ports and sockets on the front and back for attaching external peripherals. The case is well designed, easy to get into (thanks to the thumbscrews) and looks sharp with its black exterior. Remember, if you plan to use the processor from an old PC, you will have to buy a case that it will fit into.

The processor
Each barebones unit caters for specific types of processor. To find out what processor you have, right-click on My Computer and select 'properties'. At the bottom of the dialogue box should be your processor type. Make a note of this and then choose your barebones cube case accordingly - the manufacturer should have a list of which processors are compatible with each unit.

You should also be aware that the latest Intel or AMD processors run extremely fast and therefore hot, and will need to be cooled better than older chips. To reduce the risk of overheating, use a processor with a clock speed of no more than 2.8GHz (a Pentium 4 2.8GHz processor is available for £128 from CaseTech).

Before you start opening cases and touching things, though, it's important to have a set of tools at hand. Also ensure that your case is on a flat, dry surface.

Removing the processor from an older PC is easy enough: just pop open the clips that hold the processor fan in place and use the built-in lever to eject the chip. Getting ours into our cube was a bit fiddly as there's not much room to manoeuvre but most cube cases allow you to remove the top part of the chassis cage.

When fitting a processor, you should look for either the flattened corner on the chip or a small arrow and align it with the matching corner or arrow on the processor socket to ensure it goes in the right way. Raise the lever and carefully drop the processor in place, making sure all the pins are in their respective holes. Then snap the lever back in place and reattach the processor heatsink correctly, following the manufacturer's instructions if necessary.

The memory
With the processor out of the way, the next step is to add memory. If you're planning to install Windows XP, we'd recommend using at least 512Mb. We salvaged two 256Mb modules from our old PC to fit in the two free memory slots in our cube case.

Installation is simple: just push back the clamps on either side of the slot and insert the memory. You might need to be a bit forceful but they can only go in one way so you can't fit them incorrectly. Dabs.com sells individual 512Mb modules for about £49.

The hard disk
One misconception when transferring a hard disk from one computer to another is that everything will work on the new PC just as it did on the old one. Unfortunately, most operating systems don't see it this way so, if you're planning to use your old hard disk in your new cube, our advice is to reformat it and start again.

This may sound long-winded but it will mean a fresh start, free from fragmentation and any possible errors. You will need to back up your documents and files first but this should be relatively straightforward if you have a recordable CD or DVD drive or an external hard disk.

You can also make use of Windows XP's Files and Settings Transfer Wizard, which will guide you through migrating important files to another hard disk using a variety of methods.

Fitting the hard disk into the cube is straightforward as you can remove the cage and work on a flat surface. Each case manufacturer will supply instructions on how to do this. Once screwed in, the hard disk needs to be attached to the motherboard and the power supply. Our case was already fitted with an IDE cable, which only needed to be plugged into the back of the hard disk.

If your case doesn't have this feature, it's still very simple - just plug one end of your IDE cable into the motherboard socket labelled IDE1 and the other into the back of the hard disk, ensuring the pins and holes match up.

The second job is to attach the power cable. Protruding from the case's power supply should be a four-holed rectangular plug that will fit into the four-pin socket on the back on the hard disk. And that's almost it; just try and keep the cables neatly tucked away so they can't touch any other components.

The only other concern is ensuring the hard disk jumper is set to 'master' mode. This is done by following the jumper diagram on the hard disk itself and fixing the small plastic grip to the necessary pins. We used and reformatted a 40Gb hard disk from our old PC but as prices have dropped considerably over the past few years, you can now pick up an 160Gb model for as little as £59 at Misco.

The optical drive
Like the hard disk, an optical drive can be fitted quite easily if you remove the cage that the drive is housed in. Our old computer was only fitted with a recordable CD drive so we decided to treat ourselves to a dual-layer multi-format recordable DVD drive, the PX-716A (£105) from Plextor.

Once the drive is screwed in and the cage is replaced, a couple of cables need to be added - another four-holed power cable (as described within the hard disk section) and another IDE cable, which runs between a second IDE port on the motherboard and the back of the optical drive.

If, like us, you have an older CD-based drive, it's worth investing in a DVD device as they will future-proof you against DVD-based software, allow you to play DVD movies and let you record up to 4.7Gb of data on one DVD if you opt for a recordable device.

The graphics card
Your choice of graphics card will largely depend on whether you want to play 3D games, operate dual monitors, or do any other high-intensity graphical work such as video-editing. If so, then you'll want to make use of the AGP or PCI-Express slot on your cube's motherboard. We already owned a pretty decent Hercules 128Mb nVidia GeForce 6600 card (£101 from Micro Direct), so we were able to use this.

However, if all you plan to do is surf the net, use office software and look at some digital photos, your motherboard's integrated graphics should be suitable. Just plug your monitor into the VGA port on the back and you'll get an acceptable picture.

Fitting a graphics card is quite easy as most modern chassis have a flip-up security panel that will contain the graphics card without the need for screws. If this is the case, just pop off the blanking plate (if there is one), push the graphics card into the required slot (AGP or PCI-Express) and close the security panel.

Depending on the size of your graphics card's fan, you might find little room to add any additional components to free PCI slots, but these days most PCI-based hardware can be bought as external devices that plug in via USB 2 connections.

The operating system
If everything has gone smoothly then the last bit to sort out is the operating system (OS). It's possible to install an alternative OS such as Linux Mandrake (£60 from Mandrakesoft) but we'd advise sticking with what you know and settling for Windows XP Home (£161 from Amazon).

If you are building your new cube PC with bits from an old computer that had Windows XP installed, you are permitted to install it on the new PC providing it is deleted from the old one. If you bought an OEM computer, it's unlikely you will have this, and OEMs are not permitted to provide a full copy of the software with the PC when they sell it. You may have a CD labelled 'Windows Recovery Disc', but this is not a full copy of XP.

Many people find this irritating, and we think Microsoft should develop a way for people to transfer legitimately owned copies of XP from old to new PCs, but this isn't currently feasible because of the way Microsoft identifies copies of XP during product activation.

When we asked the company about this, it said: "Each copy of Windows XP is basically designed to live and die on the PC it was bought with." It means you'll have to splash out for a new copy but if the old PC had a copy of Windows 98 or 95, that might not be such a bad thing.

To start the Windows set-up process, first boot the computer from the CD-ROM drive. Once Windows is installed, the original files, documents and folders that you backed up before dismantling the old computer can be transferred to the new disk and copies of software reinstalled.

You'll also need to set yourself up with basic Windows security, such as switching on the integrated Windows Firewall (this can be found in the Security Center section of the Control Panel). You'll also want some anti-virus software. Anyone using Linux can make their own security arrangements, but a firewall and anti-virus applications are the bare minimum required.

DIY computing
If you've got a collection of parts ready to use and some spare cash, only a small leap of faith - and a few hours work - are required to transform them into a stylish and space-saving cube PC. We used a combination of old and new components and spent £300, but it can be done on a much tighter budget.

We know from our letters and emails that thousands of readers have successfully built their own PCs, so drop into our Readers to the Rescue forum for pointers if you get stuck. And remember, building a PC is far more satisfying than buying one.