Actually, you can upgrade to Vista without invalidating your XP MCE key.
Follow this workaround from Microsoft's internal documentation
Quote:
1. boot with the windows vista upgrade dvd.
2. click "install...
1. It won't unless you share files between host & guest OS
2. Yes. The guest OS is a fully functional OS on its own. Actually i have VISTA as host and XP as guest to run my legacy applications