Spybot - Search & Destroy searches and removes spyware, adware and other malicious software on your computer.

Spybot is extremely easy to use and gives a detailed description with links of the threats it finds. Updating the definitions from the web is also very easy. For those who don't have access to the internet can import updates on a separate file. You can set Spybot to run on the background, monitoring your system all the time, or like a normal application.