All it realy does is add support for XP SP3 and Vista SP1 (before it used to moan about being an unsupported OS), not sure if Win server 2008 as a guest os is new??
Just remember before you update (if you do, I'm still running the old one on Vista SP1 ok, it only moans when you install it), to make sure you shut down any virtual machines, i.e don't have them in a saved state else they won't load next time!What's new in Virtual PC 2007 SP1
This release of Virtual PC 2007 SP1 introduces support for the following:
Windows Vista® with Service Pack 1 (SP1) Business, Ultimate, and Enterprise operating systems as a host operating system
Windows Vista with SP1 Business, Ultimate, and Enterprise operating systems as a guest operating system
Windows Server® 2008 Standard as a guest operating system
Windows XP with SP3 as both a guest and host operating system
Microsoft Virtual PC 2007 SP1 download