Wendell, why no WIN 7 on the VM? Cost?
Honestly, I cannot tell you why, never really thought about it. I have a laptop and a desktop that I can use for testing. My desktop is Win 7 Pro and I DO have XP loaded on Virtual PC but never put AC on the that VM. I do run test programs that I am not sure of with it, I guess I have always trusted AC more and thus did not put it on Virtual PC.
Cost is only a semi factor, I could always reload Win 7 every 30 days if I really did not want to buy another license.
Although I have been using Win7 Pro's Virtual PC and the free Virtual PC, I really did not get into virtual machines that much until about a year ago. I needed to clone my old P2 450 machine that had an old immunization program and deactivate that machine. I then started playing with VMs. That machine was Win 2000 and no longer will run AC

Actually ran fairly quickly because Win 2000 has a fairly low overhead on the machine. Oh well, time moves on .....
Probably better to run AC in a VM, but never really worried about it. Certainly quicker to start it directly than to open a VM. Now that I am have an 8 core machine VMs are my friends.