If you haven't installed or changed anything (think, are you sure) then either a driver got corrupted or hardware is failing. Did you install SP2 recently ?

Try pulling all your PCI cards out, go into your bios and disable all the on board items and see if XP starts up. If XP starts then go back in bios and enable things one at a time and re-boot till everything works or stops. If it keeps working then install the PCI cards one at a time and re-boot.

If you find the item causing the problem then download the latest driver and reinstall.

Let me know because I have other things to try