There is a new update for Windows Vista virtual machines to enable the use of seamless applications with Windows Virtual PC.  You can download it from here:

http://www.microsoft.com/downloads/details.aspx?FamilyID=26a2de17-8355-4e8d-8f33-9211e48651fb

The big thing about this update is that it now works if you are running Windows Vista SP2 inside the virtual machine (the previous update only worked with SP1).

Cheers,
Ben