Unprotected PCs hit by viruses in minutes

The 'survival time' for an unpatched PC connected to the internet averaged 20 minutes in 2004, compared to forty minutes the year before.

The report said that users of broadband or poorly secured public networks would be infected much more quickly, in some cases under ten minutes after connecting.

'The main issue here is of course that the time to download critical patches will exceed this survival time,' the researchers said.

Security companies also monitor the state of play and are even more pessimistic.

Symantec estimate that it could take seconds rather than minutes to lose control of an unpatched PC.

'Blaster is still the largest source of these sort of attacks,' explained Tony Vincent, lead global security architect at Symantec Managed Security Services.

'It's like space junk; everything we've launched from the Earth is still up there in orbit, circling. These attacks are all still out there on the internet due to unpatched serves and never stop running.'

Symantec runs a simulated network that is left poorly protected in order to track the methods used to enter it. It has found worms written three or four years ago still in circulation.

Once worms infect machines the host PCs can be used to build networks of zombie PCs that send out spam, or launch distributed denial of service attacks against web servers.