I use Ad-aware and Spybot Search & Destroy - both have free versions. I run both because they seem to find different things. I also have Norton Anti-virus but it's not free - although they do have a link on their website that will check your computer for free - at least that way you'll know what you've got:
http://security.symantec.com/sscv6/defa ... &venid=sym
when you click on "Go" it takes you to this page (I wasn't sure if just posting this link would work):
http://security.symantec.com/sscv6/home ... rue&bhcp=1
Good luck.