How to turn XP SP2 into SP3 or even more

Start> Run> type in 'regedit'
Go to HKEY_LOCAL_MACHINE, then SYSTEM, then CurrentControlSet, then windows
From here on in click "CSDVersion" and click modify, change "0x00000200" (SP2) to "0x00000300" by typing 300 in the field where 200 once was.
Close it down,
Restart (MAKE SURE YOU RESTART)