First off Windows ME was a total bitch to do anything with PERIOD.

Second going from say Win 3.11 to Win95 was also challenging.

The biggest problem going from XP to Vista will be the change in filesystem format because iirc Vista drops the NTFS filesystem all together for another one.

Oh that and XP installed was only 1.5-2gb of room where Vista is like 15-20gb for the install so you'll also have to make sure you drive has the extra space