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Reboot - Windows Boots without warning!
By default when WinXP encounters a system failure, it reboots without warning. The setting that controls this can be changed:
Control Panel/System/Advanced/Settings (Startup & Recovery)/System Failure/Uncheck-Automatically Restart. To view the event log: Admistrative Tools/Event Viewer or Start/Run/eventvwr.
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Another very common reason is > the cpu is overheating , make sure the fan is working & if it is , try running an external fan into the internals , just to see if the problem goes away .
If it does , try cleaning the blades of all fans , including the power supply .
Make sure the power is off .