Yes...and it took nothing more than plugging in a ATA 100 HD to do it!
I have read that sometimes WinXP will force a lower UDMA.if it gets too many errors.
The fix is to "Remove" the primary IDE channel.., then the secondary, then the Main Controller ...(in the "MANAGE" hardware area.
Reboot and it will detect "New Hardware"...install the drivers and let ATA 100 work again..
If it happens to back down again you have problems somewhere in the system..