It scans the system in known places where spyware/malware/etc use their techniques to mess you system up, and gives an option to disable particular entries it finds. Since these techniques can also be used for perfectly valid programs though (eg Yahoo/Google toolbar, CD burning programs, and other stuff you set to run on startup), there'll be a lot of stuff it will list that isn't spyware, so you need to know what's what or Google extensively to filter things out
Just another tool to see what's installed itself in unusual places, I suppose, but it does it pretty well