Most spam generated by botnets, says expert

And according to anti-spam experts, Florida-based spammers are booking air time on Russian owned botnets to send their junk mail.

"Each group of spammers has a botnet," said Steve Linford, CEO of Spamhaus. "As fast as the FBI is shutting them down, others are popping up."

Linford said that every week more than 100,000 PCs are recruited into botnets without the owner's knowledge.

A botnet is a collection of -- usually -- Windows-based PCs that have been stealthily taken over by malware. Users have no idea that their computer has been corrupted.

Malware, such as Trojans and viruses, can execute on a computer either through email or by visiting certain Web sites. Often, Linford said, hitting the unsubscribe option on an email directs a user to a Web site that installs a Trojan on a computer.

Botnets send masses of spam until they are blacklisted by anti-spam firms. Linford said that once blacklisted, the owner sells the botnet to people who launch denial-of-service (DDOS) attacks.

Linford said that Spamhaus had been targeted in at least 22 DDOS attacks.

Joining a botnet to send spam is easy, added Linford: "It's easy; you just have to get into a spam club. Some of them you can enter because the security is poor. Just turn off Javascript and you can get in."

Spam clubs also advertise lists of botnets on hire and fresh proxies -- computers that have recently been taken over.

According to MessageLabs, spam accounted for 70 percent of all email last month.