Spy Sweeper

As ahas been mentioned, Spybot S&D is the best way to go. That and avast for things like viruses and you should be pretty good to go. Plus when you get something those programs can't remove the people over at Safer Networking have people who'll help you fix it.

Spy Sweeper for a long time was the top recommended Anti-Spyware program - but today it has become a flop! Not very effective like it used to be - the times how it used to remove the bigger issues of homepage hijackers, harsh trojan horse viruses that were not simple to delete, also desktop/wallpaper hijackers, and fake anti-virus/anti-spyware programs that turn out to be a virus/spyware. I recommend Trend Micro Hijack This - a small simple program that you can scan and gives you a list of files that might be infecting your computer - if you see a list of BHO - most likely there are problems of files that are infected, but not all of them. That's why whenever you come across doing a scan with Hijackthis - you can always copy/paste and send it to a tech forum for help or message it to someone that can give you help on what to remove or not. For anti-spyware I use Norton Antivirus 2007 or 2008 - the newer versions have become more sufficient into removing the tougher files. I have used AVG Anti-spyware before but it does not remove the tougher files - malware, desktop hijacker malware.