I had a problem with the XPS viewer, but installing and uninstalling the XPS
Essentials Pack fixed that. Anyway, I had changed the file association while
trying to figure out what was wrong, and doing so stopped XPS thumbnails
from being generated. Since there isn't an option to revert the file type to
its default settings in Vista, I was wondering if there is a registry key
that controls this behavior.

Failing that, if someone could post the default contents of an export of the
HKCR\.xps branch, that might help.