Your situation is common and understood
XP will not allow a CD containing an earlier (not as many critical updates included) version to repair or change a more current version. Thus, anyone current through SP2 on the system...must use a CD with SP2 included for various functions (repair install, sfc, etc.) which call for insertion/use of the XP CD.http://icrontic.com/articles/slipstreaming_windows_easy
The above link seems to be an easy way to make a slipstreamed CD, according to many.
You need that code for activation...you can get it by using a little tool called SIW System Information for Windows - http://www.gtopala.com/
I suggest you write it/maintain somewhere that is suitable.