SAN FRANCISCO -- FBI and Hallmark Card officials Wednesday issued a warning about fake Valentine Day E-greeting cards claming to be from a family member, friend or neighbor but instead contain a destructive "Trojan horse virus" that hijacks computers.

The bogus email directs the recipient to click on a link within an email to retrieve an electronic Valentine's Day card. Once clicked, the email downloads a file with a ".ru" address to your computer hard drive, converting the computer into a part of a Storm Worm botnet.

According to officials, a botnet is a network of computers that, unbeknownst to their users, forward transmissions, such as spam, viruses and malware, to other computers linked to the Internet.

"If you unexpectedly receive a Valentine's Day e-card, be careful," the Federal Bureau of Investigation said in a statement. "Be on the lookout for spam emails spreading the Storm Worm malicious software (malware)."

"The Storm Worm virus has capitalized on various holidays in the last year by sending millions of emails advertising an e-card link within the text of the spam email. Valentine's Day has been identified as the next target."

The Hallmark.com website listed the following ways you can tell whether or not an alleged Hallmark e-greeting card is legitimate.

• The subject line of legitimate E-Card notifications from Hallmark will say, "A Hallmark E-Card from (name of the sender)" not a generic term like "friend," "neighbor" or "family member.
• The e-mail notification will come from the sender's e-mail address, not Hallmark.com.
• The notification will include a link to the E-Card on Hallmark.com as well as a URL that can be pasted into a browser.
• The URL will begin with http://hallmark.com/ followed by characters that identify the individual E-Card. Hover your mouse over the words "click here" in your e-mail. If you do not see the URL above, it is not a legitimate Hallmark E-Card.
• Hallmark E-Cards are not downloaded and they are not .exe files.
• In addition, Hallmark.com will never require an E-Card recipient to enter a user name or password nor any other personal information to retrieve an E-Card.