I agree with LA. I actually run 4. AdAware, Spybot, Pest Patrol,...

I agree with LA. I actually run 4. AdAware, Spybot, Pest Patrol, and WebRoot SpySweeper. They all miss some that the others seem to pick-up; and you need to update definitions at least wkly. There are many of these programs out there and others will be just as good as the popular ones.
Spybot is good as it detects changes in registry entries or browser home pages and asks the user to allow or deny the change.
WebRoot is good as it allows you run it on pc shutdown so you can walk away and the pc will shutdown after the sweep and fix.