I to have a AMD system (athlon1.6) w/ Xp Pro and I would advise you to add another 256 DDR to the system. Do an Error Check. Does it automatically reboot without warning? You go to system/advanced/performace settings/advanced/ set both buttons to Programs then change virtual memory make sure it is set to System Managed size. Also in system/advanced/startup and recovery/settings/uncheck automatically restart - so now you will get a blue with an error code you can look up. You could have an overheating problem (check CPU temp) or a weak PSU.

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