boot to safe mode (hit F8 when rebooting). Run AV and spyware scan. Make sure there are no virus or any worms - some of them can make the system run at 100% resulting in overheating while other can insert a small code that can cause shutdown  (quite simple actually, the shutdown command, a few bat files and presto ........)

If nothing happens, go with a reinstallation and even after that, buy a new cooler.

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dump windows, linux is best