Were they actually downgraded, or wiped and a clean install?
I never "downgrade" nor would i ever in a millin years consider an upgrade. It's ALWAYS wipe and install from scratch. Just too many really really bad things happen when you install any OS over another.
Vista and Xp both come in 32 and 64bit flavors.
I wonder if the "downgrade" caused some issues - such as some registry keys not being changed and that's being seen by the install.
My advice to anyone looking in - never ever upgrade, never ever downgrade. Always format and install fresh. Doing anything else means you inherit alll and any issues that existed before the upgrade/downgrade, not to mention some keys do not get changed and it simply causes a slower computer and confusion with file versions and registry keys. IT's just plain bad advice to upgrade over the top or downgrade.
Wipe - format, whatever, but don't pop in a CD/DVD and install over the top of an OS.
I've been at this since about 1986 and no good ever comes from it, usually bad.