WindowsXP Professional Registration

Several years ago I purchased a Custom Built desk top with WinXP pro installed. I am unable to download updates because vendor must not have registered WinXP or registered under their name. I purchased a new WInXP pro to register under my name and tried to load it so that I can register a new valid ID number, it will not install WinXP unless I format hard drive which I have a lot of data I dont want to lose.
How can I upgrade my current WinXP without erasing curerent one on my computer and register my new WinXP Product ID number?
thank you