Avg is free. If you download this, you should uninstall your other virus programs. You should run a defrag once a week and set your virus programs up to run auto. I have mine set to run every day at 6. CCleaner runs at start up.
http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html (make sure you find the free one)
http://www.ccleaner.com/download (cleans files)
http://www.download.com/Spybot-Search-amp-Destroy/3000-8022_4-10122137.html
I use these and haven't gotten a virus yet.