not sure you can uninstall service pack 2. Windows put this in when you buy the computer. Usually XP home edition, It's useless we know, just takes a hell of a lot of room.
I would think a full format. save all the data you want and then put windows xp pro in.delete any partions then format and install windows. choose manual downloads for windows or it will install service pack 2 if left on auto download
you can get a tool from microsoft to change the key, or if your wanting to change your key because it isnt a valid copy, there are other ways to gt the tool