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.
Not all spyware programs run in the background. Most, at least the ones I use, are just scaners, you open them when u need them, when your done you close them and they do not run in back ground.
I use.
AVG
Aware SE Personnel
Spybot Search a& destroy
SpywareBlaster
Microsoft Antispyware Beta
of which AVG and spybot are the only ones that run in background.