Oh that everyone's drive can only be changed a limited number of times its just one of those stupid anti copyright things. Download
AnyDVD if you want to change it beyond the limit. You put a disc in your drive and it changes the region automatically, works a treat. You can try the mentioned app for 21 days after which you have to cough up some £, I'm not aware of any free apps that does the same thing maybe someone else may be able to recommend a good free alternative.