Reformating with Windows XP

OK, I'm wanting to wipe my hard drive and reinstall my OS and everything. The reason being is that I downloaded an update for Windows that I don't like and eventhough I think I know which one it is I can't seem to remove it. So, I know how to reformat and reinstall my OS-but my question is how will I reregister XP. Since, I have already registered it-can I unregister it or what. I'm affraid that if I delete my hard drive and reinstall windows I may end up not being able to register XP (since it is already registered-eventhough it will be on the same computer). Does anyone know how to take care of this-or if it is even a problem?