In addition to Norton Antivirus, I’ve used Webroot Spysweeper for a long time, but have grown tired of how it slows my systems down and uses so many resources. I finally took it off my main computer and now rely on a couple of free tools to fight spyware. These tools are Spybot Search and Destroy and Lavasoft’s Ad-Aware SE Personal. Both do a great job of scanning your computer for spyware, and Spybot adds filters that keeps you away from known dangerous sites and provides other immunization.
Since I don’t mess with downloaded games, stay away from questionable sites, never activate questionable files attached to emails, and avoid other bad practices, I usually don’t get affected with malicious spyware and find the free tools to be adequate for my needs.