Virus writers are increasingly targeting poorly protected home PCs because company defences are proving too much of a challenge.

Vincent Gullotto, vice president of the Anti-Virus Emergency Response Team (Avert) at security company McAfee, said recent attacks have ignored corporate networks and aimed for the home user instead.

"The [corporate] perimeter is no place for success these days [and] 80 per cent of virus submissions now come from home users," he said.

"When a home PC is bought, the software on there is usually three months old and needs updating - but many users don't realise this."

Microsoft's Windows XP Service Pack 2 (SP2) might help with this, he added, but with the slow pace of updates it could take six to nine months before it really has an effect.

"SP2's effect will be limited as it's not got a phenomenal set of features," agreed Professor Neil Barrett of Cranfield University's computer science department.

And in the longer term, predicted Gullotto, security firms will no longer be able to rely on signature files, which are updated centrally, to identify individual viruses and kill them.

Instead, protection software on each PC would have to analyse suspicious behaviour and act on that.

"While antivirus protection is still strong it's not going to cut it all the way," said Gullotto.

"In five years we'll see more success from behaviour-blocking analysis than we do, and this will be integrated into future technology."