Windows Vista trial virtual hard disk
This is nice - a 30 day evaluation version of Windows Vista Enterprise, pre-installed with Virtual Machine Additions already loaded. You can download it from here:
Note that while the download page says that you need Virtual Server, the virtual machine works fine with Virtual PC 2007. The virtual machine is configured with 768mb ram and a disconnected network card - so you may want to fix that up before you get going.
Cheers,
Ben