Upgrade or Full XP

[Tinker]

Well then..... You may be the recipient of operator error from the factory, that is you could gotten a copy of the full version put into an upgrade packege.........

Look for a file called Setupp.ini that is present on your Windows XP CD and double click on it and browse its content. There is a field called PID=. This PID is what would tell us exactly what we have got.First 5 bytes of PID decide whether (it is OEM or RETAIL or is an UPGRADE ) and last three digits determine what kind of CD KEY will it accept. Here are some of the typical PID values:
Retail = 51882 335
Volume License = 51883 270
OEM = 82503 OEM

Source: XPTuneup Tip 7