For Virtual Machines, look at Windows Virtual Machine or VMWare.

Craig, just a note, I've been using Sun VirtualBox for a few months now, and am very happy with it.

LIke the free version that comes from MS, VirtualBox is also free. However, VirtualBox has more functionality and much better 64-bit support.

I used VMWare for quite a long time. It's a great product - but for many developers, VirtualBox works fine. (While there's no official benchmark to support this, some have commented that VirtualBox actually "seems" a tiny bit faster than VMWare. My own naked eye sense, so far, is that I can't tell the difference in performance. What's significant is that they could be in the same league).