Every year around this time, spammers send out millions of spam related to Valentine's Day. This year, they're making a particularly strong effort to regain lost ground.

Spammers are trying to regrow their botnets (define) to make up for the loss of botnet host McColo, which was taken down in November. That shutdown slashed spam levels worldwide by up to 70 percent.

"The amount of Valentine's Day-related spam we're seeing this year is 50 percent higher than last year," Adrian Duigan, product marketing manager at Web and e-mail security vendor Marshal8e6, told InternetNews.com.

The spammers' increased activity bears out predictions made last month by Google (NASDAQ: GOOG) that spammers would try to regrow their botnets this year.

As they always do around this time of year, spammers are sending out e-mails with subject lines containing keywords such as "Valentine's Day," "February 14," "Love," or claiming the recipient has received a Valentine's Day card. These e-mails contain links that either serve up advertisements or redirect the victim to another site where malware is downloaded to take over the victim's computer.

Some sites have pictures of hearts or puppies and ask visitors to guess which one is for them. When a visitor clicks anywhere on the site, it downloads Trojans named onlyyou.exe or youandme.exe, which can connect to remote command and control Web sites and receives commands from them or sends information to them about the victim's PC, Carl Leonard, threat research manager at Web security vendor Websense, said in an e-mail.

Or, they post links on victims' blogs or social networking sites such as Facebook, Twitter and MySpace. They also send out e-mails to people purporting to be from their friends' social networking sites. Clicking on these e-mails downloads a banking Trojan which will steal the victims online banking login credentials, Leonard said.

This article was first published on InternetNews.com.