Spontaneous rebooting

My pentium [I] [166mhz 80mb ram and 2gb harddrive with WindowsME] with windowsMe is about 5 years old. Recently it has been for no apparent reason spontaneously rebooting itself. The display settings are at 16 bits [sometimes I changed it to 256 colors with no improvement] and 800X600, and moved the advanced performance scale all the way down. I also have reset the power management by removing hard disk turning off [it just has monitor turn off every 15 minutes or so]. And I have disabled the power savings thing in the Bios. But nothing seems to help. Is there a solution to the problem or is the computer [power supply, battery etc.] dying?

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