DirectX 8.1 Reinstallation

My OS is Windows XP pro. After reinstalling my Nvidia G3 Ti 200 drivers, I have come to the conclusion that somehow, my DirectX 8.1 is corrupt. Anytime I try to play an mpeg or game, my PC just crashes or reboots itself.

What is the proper way to reinstall DirectX 8.1 on my OS?
Pleas help!
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