When you say, "stalls at the Windows XP screen", what exactly do you mean? Does it get to the `Welcome` screen, do you get to the loading screen (the one with the moving bar in it) or do you get to a blue screen with perhaps writing on it?
Have you left the PC on for any length of time to see if it does start?
When it "switches off" does the screen go black or is there an indication of signal? What do the power and HDD lights (normally green and red respectively) show during the start to switching off sequence?
Have you tried booting from a CD/DVD such as your XP disk? If not you try this to see if it boots satisfactorily as it will eliminate a lot of possibilities.