SpyBot (use google) is quite good... update the information file prior to scan. It scans for around 12,000 known spyware, allowing you to delete, replace, modify the file system and registry accordingly...
I use two programs. Ad-Aware does a great job most of the time, but I've found that I also sometimes have to run Spybot to get the few programs that Ad-Aware misses.
I must wholeheartedly agree with Sir Whiskers--run both Ad-Aware and Spybot S&D if you can... The two of them together catch just about everything and anything.