Your computer repeatedly restarts after you install Windows XP Service Pack 2
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SYMPTOMS
After you install Microsoft Windows XP Service Pack 2 (SP2), your computer repeatedly restarts. You do not receive an error message.

CAUSE
This problem occurs if all the following conditions are true:
You installed the 32-bit version of Windows XP SP2 on a computer that supports hardware-enforced data execution prevention (DEP), such as a computer with an AMD 64-bit processor.
You installed hardware that requires the Mpegport.sys driver.
When you restart your computer, the Mpegport.sys driver tries to run from a memory location that is monitored by DEP. You receive a Stop error that is similar to the following:

0x000000FC (ATTEMPTED_EXECUTE_OF_NOEXECUTE_MEMORY)

Your computer may restart immediately if it is configured to automatically restart when a system failure occurs.

More to read:
Microsoft Knowledge Base Article 878474