A computer virus masquerading as a digital postcard could install a Trojan, or worm hijacking software on your computer, allowing others full access to your PC.

Simply clicking on the link contained in the e-mail message launches the malicious software (malware). "The bad thing is it opens what they call a back door and what that will do is allow someone at some point to exploit your computer in any number of ways," computer technician Dan Blanchard told NBC News.

The e-mails look innocent enough. The subject lines say, "You've received a postcard from a family member," or "holiday e-card," or "school friend sent you an ecard from postcards.org," or "You've received a Hallmark E-card!" Do not click on the link to retrieve the card; it is infected with a virus.

"See if the address given matches the other one in the e-mail - and in this case it does not," Blanchard said. "If you click on it, which you probably wouldn't want to do, but if you do, it will bring up (an install program)."

The FBI issued a warning a couple of days before Valentine's Day, which is apparently a big day to distribute viruses. "The Storm Worm virus has capitalized on various holidays in the last year by sending millions of e-mails advertising an e-card link within the text of the spam e-mail."

PC World reported, "The Storm Worm is a botnet that turns your computer into a work horse for the controllers of said virus, allowing your machine's power and Internet connection to be utilized for spam, identity theft, denial of service attacks, and more."

Experts advise never opening unsolicited e-mail; delete it and then delete it from trash. Don't open any e-mail with a ".exe" attachment. Make sure your virus scanning software is current and running. Update your operating system – especially Windows – with the latest patches to plug security holes.

If you have received an e-mail like any of the ones described in this article, you can file a complaint with the Internet Crime Complaint Center (IC3), a partnership between the FBI and the National White Collar Crime Center (NW3C).

The BBC reported recently that malicious code in circulation on the Internet increased by five-fold in 2007, compared to 2006, and one technology security company says it saw 3,000 different pieces of malicious code a day.