..... Meant to tell you guys about this a month or so ago.
It seems that a lot of folks either want to run Vista in a VM instead of installing it outright on their computers, or they have upgraded to Vista and want to run XP in a VM so they'll have access to some old applications that don't work with Vista. You'll be happy to know that Microsoft's Virtual PC (VPC) VM application is now a free download. You can install VPC 2004 on XP and run Vista in the VM, or you can install VPC 2007 (beta) on Vista and then install XP in the VM.
http://www.microsoft.com/windows/virtualpc/default.mspx