I ran Avast for a few years, but dumped it awhile ago - too heavy for my system. AVG runs significantly lighter for me. But... I've come around to realising that you tend to get what you pay for with antivirus software, and both of these tend to score quite low on the comparative tests - I think Avast is a bit higher than AVG. If you're running free AV, be sure to have other layered security programs - things like a2, MS AntiSpyware, Kerio Personal Firewall, etc. As I say, I used free security programs for a long time - now I swear by NOD32 and Outpost for myself, and for others who want to stick with free things, usually a combination of AVG and Kerio for the lightest resource-usage.