Internet users are being warned to stay vigilant by researchers who believe that next Monday could be the worst day the year for computer attacks.

After analysing information on viruses and internet worms taken from more than 500,000 machines around the world, security experts at PC Tools have pinpointed November 24 as the potential peak of malicious software activity for 2008.

Data from 2007 showed that the high point of action from viruses, worms and other internet-based attacks came three days before America's Thanksgiving holiday, leading them to suggest that the same day could prove the bleakest 24 hours of this year.

The company says the increase in virus activity could be the result of internet shopping in the run-up to Christmas, as millions of users begin going online to purchase gifts for their friends and family.

With the chance for criminals to access financial details, online shoppers make an attractive target for the writers of malware.

The number of people shopping online this Christmas is expected to grow again this year, with internet sales in the UK alone predicted to hit £13.16bn �?an increase of 15% over 2007.

More people are expected to use the web for their shopping as a way of finding lower prices to help them beat the credit crunch.

But while security experts are expecting the traditional increase of malware over November and December, they have also warned of another source of potential attacks. Spammers and criminals have attempted to use the popularity of American president-elect Barack Obama as a way to trick users into downloading malware.

A flood of emails purporting to link to a video of Obama's acceptance speech were actually connected to a trojan horse program which would compromise the target's PC. Meanwhile, a separate burst of spam messages contained a file called BarackObama.exe, which, when activated, places a rootkit on the victim's machine and opens it up to hackers around the world.