NoAdware is a decent adware and spyware scanner and removal tool. Like most of its competitor software, it will scan your machine for free, and then require purchase to allow removal.

The company rightly claim that your PC is likely infected with Adware or Spyware if you have:

downloaded music online
noticed you PC is running extremely slow
noticed you are pestered by those horrible popup ads
noticed that your homepage keeps changing

NoAdware has always lagged behind the more popular SpywareNuker in our poll, but the software seems robust and is not a second rate choice.