I know many developers who rely on Virtual PC 2004 these days, both for application compatibility testing purposes and also for beta testing releases of Visual Studio 2005 ("Whidbey") and SQL Server 2005 ("Yukon"). A few colleagues and I have been trying to track down a little issue for some time, and I wanted to share it more broadly outside of Microsoft to see if anyone else has had the same problem.
The problem is this: when you toggle a Virtual PC guest into full screen mode with [Alt Gr]+[Enter], the [Ctrl] key often gets "stuck" down. If you start typing in the guest, you often see funny things happen: the taskbar goes into multi-select mode; some characters don't appear etc. Hitting the [Ctrl] key once resets it and brings you back to normal.
A few of us raised this as a bug with the Virtual PC team, and it turns out to be a problem specific to UK keyboards (and possibly others in Europe). The [Alt Gr] key is effectively the same as pressing [Ctrl] + [Alt] together; when you release it, VPC only toggles off the [Alt] portion of the keystroke, leaving [Ctrl] held down as far as the guest is concerned.
The solution: change the host key to something else on a UK Virtual PC machine. I can recommend [Right Ctrl], since it's not used for anything else. Once you make this change, you'll never again have this problem - toggling to and from full screen works just fine. The bug has been fixed moving forwards too, which is good news.