Webroot's Spy Sweeper is a pretty good one. It looks for adware and spyware. You can get a free trial version from
www.tucows.com. You can't update the scan files without subscribing and a subscription is $29.95 a year. That's cheaper than computer repairs though! You do have to be careful and read about what it finds. One of the things it identified as Spyware for me is actually part of Turbo Tax. It warned me though that Turbo Tax might not work if I deleted that file.
You'll be pretty amazed the first time you run one of these programs if you surf the net a lot.
Dee