A Guide to Adware and Spyware Removers
Ad-Aware
If you're browsing with Opera you should never need this, but if you still use Internet Explorer or Kazaa and your homepage keeps randomly changing, you get porn pop-ups and your computer generally chugs along then its likely you've got one of these (or indeed multiple - I've seen several hundred on one computer!) little buggers to thank for it. Virus checkers will pick some up, but you need something like Ad Aware to get rid of the rest (free for personal use). Don't bother installing loads of programmes - this will do you fine. Don't scan continually - once a month seems to be more than enough for me on a permanent connection (therefore at greater risk) and asides from the occasional one in my Internet Explorer temp files (yes, you can pick them up even when you're not running the browser - gotta love Windows design ;)) I have none.
Spybot - Search and Destroy
This is also recommended by many people extremely highly, however having never used it I can't comment. I'll get round to testing it sometime in the future.