I personally use Spy-Bot, as it has a really good systems protector too, called Tea-Timer, that protects changes in the registry.
It can get annoying when you are installing something, and it keeps asking you wheather you want to accept the change to teh registry, but you just have to remember to switch the program off while installing programs/games.
The only downside to Spy-Bot is the speed at which it runs, but, with the amount of spyware now, it doesn't seem too bad.
On my machine which is a 3500+, it takes around 6 or so minutes to do a complete scan.