DVD region problem

Hi there,
I have Windows XP Home Edition. I have recently changed my DVD region from
region 2 to watch an American DVD. I still have 1 change left, and yet when i
try and change the region back to region 2 it comes up with the message:

Unable to update region setting. Please make sure the drive contains region
2 media and you have administrative privilege.

I have tried several of my region 2 dvds and i am the only user and
administrator of the computer.
Any help would be much appreciated,
many thanks