Anti-virus in a Virtual Machine

I do not believe a virus on a VM can affect the host OS, but the VM itself can certainly be infected. I don't run anything other than MS security essentials on my Win7 VM, but I don't use it to do anything other than a couple of applications- certainly no email or web browsing.
I do have a couple of XP images that I keep backups of, so I can boot up a clean machine to test anything, and roll it back in seconds.