My operating system was changed

Why did my operating system change?

I own a Dell Inspiron 600 M, it origin operating system is Windows XP Home Edition but for some reason it got changed it is now running on Windows 2000 not sure how this happened but need help reactivating my Windows XP again. Is there anyone that knows how I maybe able to do that? I've already tried re-installing the Window XP operating disc and it say's I already have it installed I just don't know how to re - activate it again? All help would greatly be appreciated and welcomed.