I find the idea of passing on a virus to a PC user accidentally to be somewhat...questionable, myself. How many times are you likely to receive an email with a file attachment like "naked pictures.exe" or 'your delivery.exe" and think "Wow, it'd be a great idea to forward this to Joe!"?
I don't install AV on the Mac side. I do run VMware, and on my Windows VMs I use a Windows AV program (the Windows version of Clam, in fact), but I don't know that running an AV program on the Mac side is really the best way to protect a Windows VM.
