MADISON, Wis. -- Repair shops are plenty busy these days fixing computers overrun by "spyware."

SPYWARE If you believe spyware has been installed on your computer (one sign is pop-up advertising that is not affiliated with the site you're viewing), software can help you remove it. Ad-aware Spybot-S&D Survey: Have You Had Spyware?

Microsoft says the secretly installed programs that cause pop-ups and tracks where you go on the web is the cause of more than a third of all PC crashes these days.

"I would say it is probably No. 1," said repair shop owner Steve Diloreto.

Consumer Reports says just about anyone who goes online has spyware on his computer, but some are overwhelmed by it and can not function properly.

"Spyware is parasite software that sneaks onto your computer when you click onto a pop-up ad or download free software," said Kim Kleman of CR. "It can track where you go online and even highjack your browser."

A computer loaded with spyware can be inundated with pop-up ads -- they just keep coming. If you try to do a search, the computer slows to a crawl or freezes up.

"Spyware is a cheap way for online advertisers to target their ads or have people click onto their Web site," Kleman said.

CR just tested six programs designed to protect your computer against spyware.

Ad-Aware's free program is a good choice. Whenever you run it, Ad-Aware scans your computer and eliminates any spyware it finds. But some people need even more protection.

"If you do a lot of downloading, you should run a program that continuously scans for spyware and prevents it from ever getting on your computer," said Dean Gallea of CR.

The $27 version of Ad-Aware provides continuous scanning. Pestpatrol does too, and costs $40.

CR says don't put off installing a spyware program. To avoid spyware, CR say to be wary of free music and movie file-sharing programs.