The FBI announced last Wednesday that more than 1 million computers, most of which are in the United States, have been infiltrated by malicious software and made part of a so-called botnet of zombie PCs. These infected PCs are then remotely controlled to distribute malware to other users, steal personal information, and perform other dubious activities.

Although the FBI can't find every infected PC or contact all the owners of these computers, it has identified and contacted some users and made several high-profile arrests in recent weeks. Sadly, this activity has led the FBI to warn users that hackers might seek to infiltrate more PCs by sending malicious email messages that pretend to be from the FBI.

"Bad guys will continue to use whatever tools are available on the vulnerable, on people who are unaware or unsuspecting," said FBI assistant deputy director Shawn Henry. "There will likely be spam sent on the heels of this case," from people masquerading as the FBI, he added.

During a sting code-named Operation Bot Roast, the FBI arrested several high-profile hackers and spammers, including Robert Soloway, the so-called "spam king." An infamous mass spammer, Soloway continued to send spam even after he lost a $7 million court case against Microsoft two years ago. Other arrested spammers include James Brewer, who is charged with infecting more than 10,000 PCs around the world, and Jason Downey, who flooded botnet computers with spam for 11 weeks in 2004 and caused as much as $20,000 in damages.

The FBI says that cooperation with international law enforcement was key in making Operation Bot Roast successful. The agency worked with law enforcement officials in 60 countries around the world.