So, what downgrades does Microsoft allow?
Owners of the OEM editions of Vista Business and Vista Ultimate can downgrade to Windows XP Professional, including Tablet PC Edition and x64 Edition. Only the OEM editions qualify for a downgrade, so if you purchased a new PC with either Business or Ultimate preinstalled, you’re in like Flynn. Those who aren’t: All users of Vista Home Basic and Vista Home Premium, and anyone who upgraded to Vista using a retail edition of any of the operating system’s SKUs. You are, as they say, SOL.