Overclocked parts.
Overclock is overspec. No guarantees, no promises, never. So an overclock which works perfectly one day might become unstable the next day, it happens eventually to every overclocked part, longevity of an overclock is as unpredictable as any other parameter of an overclock.
Although I'm suspecting the first crash might have had something to do with unstable RAM or system drive fault, while the subsequent problems are just Windows being as arrogantly stubborn and stupid as usual. I'd try booting Windows (and perhaps also a linux) off USB a few times, just to confirm whether the fixed WinOS install is somehow "broken".
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[/Korth]