Adaware used to be top of the heap, but the latest update seems to be a little old...
you might want to try spybot now as well.
http://www.spybot.info/en/spybotsd/index.html
Can work alongside Adaware. Latest update weqas issued 28th Jan this year...
While we're at it, make sure your Anti-virus is up to scratch, that should be your first line of protection against real proper nasties such as trojans etc.
AVG7.0 from
www.grisoft.com
Free and decent.