You need Microsoft Security Essentials. It's free, it doesn't slow down the computer to 486 speeds, and is constantly updated. Did I mention it's FREE? No popping up ''enter your credit card info'' like all the rest with endless upgrading and subscriptions.
It's caught quite a few Trojans and the like on incoming emails here on the shop's network.
http://www.microsoft.com/security/pc-security/mse.aspx
On edit: Oh yes, Zone Alarm. I remember THAT thing. It adds about 30 to 45 seconds to the time it takes to open a Word document. We tossed that from the machines here a long time ago. Semantic (Or Norton AV) is another virus app that throws out an anchor behind the machine, making ev... ery... thing... so... s... l... o... w.