Microsoft and Washington state are cracking down on scammers who bombard computer users with fake warning messages in hopes of selling them useless software.

On Monday the state's attorney general and lawyers from Microsoft's Internet Safety Enforcement team will announce several lawsuits against so-called "scareware" vendors, who are being charged under Washington state's Computer Spyware Act.

The vendors targeted by the lawsuits are not being named until Monday, but the Washington attorney general (AG) referred to them in a media alert sent out Friday as "aggressive marketers of scareware -- useless computer programs that bilk consumers by using pop-up ads to warn about nonexistent, yet urgent-sounding, computer flaws."

This is not the first time Microsoft and the Washington AG have teamed up to fight scareware. In 2005 they jointly sued Secure Computer, a security software company they accused of using fake error messages to scare users into buying its Spyware Cleaner software. Secure Computer eventually paid US$1 million to settle the charges.

The Washington AG has also brought lawsuits against companies such as Securelink Networks and High Falls Media, and the makers of a product called QuickShield, all of whom were accused of marketing their products using deceptive techniques such as fake alert messages.

Fake alert messages can be effective. Earlier this week, researchers at North Carolina State University reported that computer users are highly likely to click on fake Windows error messages. In their study, nearly two-thirds of respondents clicked "OK" when presented with a phony Windows pop-up message.

The use of these fake messages is a growing problem on the Internet, said Washington Assistant Attorney General Katherine Tassi in an interview earlier this week. Scammers are "getting more and more creative and putting more and more effort into making them look like security messages," she said.

The most prevalent scareware program in circulation today is software called Antivirus XP 2008, according to Alex Eckelberry, president of Sunbelt Software. Often installed on a PC without proper notification, the software bombards victims with fake security warnings, trying to convince them to buy worthless programs that sometimes even harm their PCs.