in the true spirit of technical support...
have you tried a shut down and restart ?
seriously though, have you tried pulling all peripherals and letting the system boot without graphics cards, additional hard drives, scusi drives / cards sound cards / wireless cards (if desktop), usb accessories (desktop & lappy) etc ? you can also perform a diagnostic boot in xp, which you can see line by line each part of the boot process and it hangs on one specific itme that could be your problem. i had this happen after a MS driver update to an nvidia graphics card. had to pull the card let the system boot then reinstall the old version.
could also be, as milo suggested, a mobo component on the fritz.