Did you do a clean install of XP ie did you format the hard drive,or did you install over the existing XP.
When you reinstalled windows did you install all the motherboard drivers especially ones for your video.
Is the pc overheating.
Have you looked in device manager to see if all the hardware devices are installed properly .
Have you looked at the system information within event viewer to see what errors that is reporting.
Answers to the above questions may provide clues to your problems.