What have you downloaded that screwed the pooch?
I go with Ad Aware, Spybot, and Microsoft AntiSpyware. All are free, and safe.
MS AntiSpy can hang sometimes, and pull your comp down to a crawl. If it does, boot to safe mode, uninstall it, reboot, and reinstall. It's the same as any MS product in that it can become corrupt...
You can also go to http://housecall.trendmicro.com/
and use TrendMicro's online spyware scanner. They also have an online virus scanner you can use there as well. Both are free.