Over the last couple of years, I've become a big fan of the Open Source community and those companies offering users alternatives to high price commercial products. AntiVirus software has become a huge money making business for companies like Symantec and McAfee (and countless others). At $40 – $75 per year for antivirus subscription software, it starts to get a little ridiculous on how far you have to go to protect an operating system that should be better equipped to handle virus threats to begin with (ie. Windows)!
I have been using AVG Free AntiVirus software from Grisoft for a couple of years, and have been very happy with the protection it has provided me. Also, since it just does antivirus checking and repairing, your system won't feel as sluggish as it does when using Symantec or McAfee's suite of products. Combine those features with zero cost, and you have yourself one great free antivirus product for your personal PC.
In addition to Free AntiVirus software, they also offer Free Anti-Spyware and Free Anti-Rootkit software.