Can't change DVD Region in Windows XP

[Guilhermino]

I don't seem to be able to change DVD Region in Windows XP, even though I have a disc of the region I want to change to in my drive, and all the administrative privileges ( or at least I'm an Administrator in this PC ). Windows XP returns an error message saying it is not possible. I have 2 changes left on my LG-DRD8160B drive. Can anyone help me?
Thanks.