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What if you remove the RAM? Make al different combinations if you have more than one, change them from the slots and verify they are properly installed... Also, you can try to remove the battery.
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Try installing sp4 in w2k and this patch MS03-026 (Windows2K-KB823980-x86-ENU.exe).

In XP go to run and then type shutdown -a that will stop the rebooting process, then install SP2.

that must...