Completely random computer restarts (crashes?) on any OS

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http://serverfault.com – My computer sometimes suddenly restarts. This issue has always existed, and I can't seem to figure it out :/ Have been living with this issue for more than a year, now it's time that it changes! The computer is just suddenly off, and starts up again. It's completely random, sometimes it doesn't happen for days, sometimes two or more times right after another. Sometimes right after boot, sometimes after hours, sometimes not at all. Sometimes when I am using the computer heavily, sometimes when I'm not using it at all, or only surfing, or only playing music, etc. It happens on both Windows 7 (HowTos)