No AV program installed on this or my wifes computer for over a year. I just use threatfire on both machines.
Takes very few system resources, just watch's for rootkits, registry changes, etc.
In the past I've had virus's disable my AV before it could detect it. That doesn't happen with Threatfire.
If it wants to change anything, threatfire is going to stop the process, ask me if this was something I wanted to do.
And if I say no, it will quarenteen the offending file, and kill the process.
But don't take my word for it. Check it out yourself.
Also its not constantly having to update virus definitions, etc. Install it and forget it.