I've found Microsoft Security Essentials to be adequate for VM use, as it's very lightweight. Since you're running a VM, so long as you keep the important stuff backed up then you can always just nuke it or use VMWare's snapshot tools to grab a copy of a secure version of your VM that you can restore if need-be. Any anti-virus with more power to it can bog down a VM quite badly. The only real problem with MSE is that if you're on Windows 7, which seems to love asking you to re-activate it, then it spawns extra warnings, but then that's the Microsoft way of doing things 🙂
Now though I'm mostly using WINE, it's a pain to set-up and definitely not for the faint-hearted, but it runs a lot more programs, and generally a lot faster, is similarly "sandboxed" as a VM in that only your fake Windows drives can be damaged by malicious software, and if you use Mac anti-virus then it will protect the Windows folders with little fuss.