here is the location of the PID: HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersion
but just as we said before this PID is not going to help you at all with the install of windows XP.
if you are wanting to access this informaiton in the middle of the XP install you can do this by pressing shift + F10 (this will open up a command prompt) then type in regedit, this will open up the registry editor for you, then you can access the pid from there.