Microsoft and Washington state's top prosecutor have filed a lawsuit to stop "scareware purveyors" that trick people with pop-up messages claiming computers need critical repairs.

Attorney General Rob McKenna and the US software giant filed suit on Monday against a pair of Texas businesses accused of sending invasive programs into computers to frighten people into buying a Registry Cleaner XP service.

McKenna said the lawsuit filed under the auspices of the state's Computer Spyware Act "has yanked the fear factor dial out of the hands of businesses that use scareware as a marketing tool and have spun it toward them."

The complaint accuses the operators of firms Branch Software and Alpha Red of sending incessant pop-up messages resembling system warnings to peoples' personal computers.

The messages read "CRITICAL ERROR MESSAGE! REGISTRY DAMAGED AND CORRUPTED," and instruct users to visit a Website to download Registry Cleaner XP.

The orchestrators of the campaign took advantage of a Windows operating system feature designed to let computer network administrators send notices to people using machines.

People that went to the Registry Cleaner website were offered a free computer scan that always reported finding "critical" errors, according to senior counsel Paula Selis, head of the attorney general's Consumer Protection High-Tech Unit.

Visitors were then invited to pay 39.95 dollars to have the purported computer problems fixed.

"We won't tolerate the use of alarmist warnings or deceptive 'free scans' to trick consumers into buying software to fix a problem that doesn't even exist," McKenna said.

Microsoft referred the case to McKenna's high-tech unit and helped put the case together.

"Microsoft is honored to assist," said Richard Boscovich, a senior attorney for the Washington-based company's Internet Safety Enforcement Team.

"Cybercrime continues to evolve, but with public/private collaboration such as this, we can work to champion tougher laws, greater public awareness and, ultimately, stronger protections for online consumers."

The suit demands that the "spyware purveyors" be barred from continuing the "deceptive" practice and be ordered to pay restitution to customers, civil penalties, and legal fees associated with the case.

Microsoft has filed 17 "civil spyware action" suits since the state's Computer Spyware Act was enacted in 2005. This is the seventh suit filed under the statute by McKenna's office.

Microsoft estimates that half of the computer crashes reported by callers to its customer support lines can be blamed on spyware fouling machines.

"Spyware has arguably become the biggest online threat to consumers and businesses since the advent of the Internet," McKenna's office said in a release.