1. That problem could be caused by anything from faulty hardware to a corrupt system file. Please give us as much detail/background info on the problem as possible so that we can (hopefully) narrow down the list of "possible suspects".
2. Does the computer boot successfully if you start it up in Safe Mode? To boot into Safe Mode:
Restart your computer.
When the machine first starts again it will generally list some equipment that is installed in your machine, amount of memory, hard drives installed etc. At this point you should gently tap the F8 key repeatedly until you are presented with a Windows XP Advanced Options menu.
Select the option for Safe Mode using the arrow keys, and then press Enter.