What anti-virus program should I use?

I'd go with AVG, Very good and does it's job. You don't even know it's there until it catches something.

 

Avast

I personally use avast (free home edition since 2004) on my personal machine and it hardly ever has any problems and usually avast catches anything, I also recently added MSE also. When I work on most customers computers to fix "viruses" I i notice that they usually use expensive ram hogs such as *UGG* McAfee or *shudder* Norton's and are eaten up.

All depends on if you actually use the antivirus or just expect it to protect you by itself and what you do that may get you a "virus" (such as using Lime wire) nothing is going to protect you 100% (pretty much unless you do not use the internet).

 

I like comodo Internet security. AVG failed me in the past. Comodo hasn' yet... free version too