I use AVG Free vs 8.0 on my laptop and it seems to a good job. (www.grisoft.com) I also use Spybot Search and Destroy 1.6, Wndow Defender (not that great) and Lavasoft's Adware. On my desktop I use all of the above except AVG, instead using Trend Micro's Internet Security.

I will not use Macafee nor Symantec (Norton's A/V) and strongly recommend that you never use Norton Utililties because too many friends have with disasterous results like blown hard drives.

Sometimes the best fix is just to get a new hard drive.