As Conniekat says, Format c: should help, if there's nothing physically wrong with your system.  Even better is Format c:s but random crashes are often due to overheating.

I would highly recommend you open the casing (with the power off, of course) and see if there's a buildup of dust anywhere.  It's worth removing the processor fan and cleaning the heatsink vanes with a toothpick and sweeping the dust away from the power supply vents.

BTW, you can safely use a vacuum cleaner provided you don't go at it like a bull at a gate.

Coppula eam se non posit acceptera jocularum.

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