I was told that the upgrade from Win2k Pro to WinXP Pro could not be undone just like in Win9x. I did the upgrade myself and confirmed this issue. As a result, to downgrade Windows with NTFS partitions, you have to let the Win2k setup format the partition where Windows is installed in order not to leave any superfluous files on the partition (you may instead use Partition Magic or another utility to set up the partitions).