Hi,
I use EZ antivirus (computer associate) from ages.
It's very resources friendly (important for me as my PC is a gaming machine offline and online!)
It's a annual fee of 40 $ .. but it's worth for this very good antivirus.
Very easy to set ... no a gas factory like Norton..and runin silent mode.
Never a prob over 5 years .
http://shop.ca.com/virus/antivirus.aspx
Regards.