Scores of people returning to work after the Bank Holiday could have a nasty surprise awaiting them in the form of a new internet virus.
Experts warn small and medium-size businesses are most at risk from the virus known as Sasser, which spreads to computers direct from the internet, unlike most viruses transmitted through e-mails and attachments.
The virus, which came to light in the past couple of weeks, is thought to have infected about 2,000 computers so far, although some estimates say as many as one million machines could have been hit.
It attacks recent versions of Microsoft Windows, such as Windows 2000, Windows Server 2003 and Windows XP, and causes computers to slow down and crash.
Although some experts said the virus could only be picked up from "off the beaten track" websites, such as pornographic sites, or sites where you could illegally download software, others said computers could become infected just by logging on to the internet.
Graham Cluley, senior technical consultant at anti-virus firm Sophos, said: "You can get it just by connecting to the internet, you don't have to open an email or go to a dodgy website.
"If you don't have a firewall in place there is a good chance chance you will be hit."
He said many people may find they had been affected when they went back to work today, while company computers could also be infected by people who had used their laptops to work at home over the long weekend.
He added that an email had also emerged which appeared to be from an anti-virus company and warned people they had been infected with the Sasser worm, but if people opened the attachment their computer would be attacked by a new virus.
Mark Grady, of IT consultancy Intraliant, said large companies were unlikely to be affected by the Sasser worm as their firewalls would keep it out and they would have regularly up-dated anti-virus software.
But he added: "Small to medium size businesses are more at risk. These are the people who should be checking their machines."
A security patch to protect computers from the virus can be downloaded from Microsoft's website.
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