I use Avast AntiVirus because so far it's worked great for me. It does what it should, like automatically update its virus definitions, informs me of program updates for the actual software, and can scan incoming e-mail, IM file transfers etc. Best of all it's free. You register your copy and it's good for like 14 months, I believe. Once those 14 months are up you just re-register and get another software key.