I presume you want something more like a hypervisor on your hardware, not something like Virtualbox that runs inside an O/S. I had pulled down an older VMware hypervisor but could not bring myself to install it (I despise VMware). Have not had enough time to play with Xen personally, though I have rights to muck about with my company's Xen farm enough to cause trouble.
For your Windows box, try disk2vhd from Sysinternals to pull an image to a file. I made a copy of a box just recently and it worked fine (in Virtualbox) with minor mucking about. You'll most likely need to reactivate to keep Microsoft happy, of course.
I'll be very interested in the responses you get, thanks for the question!
