Check your energy saver settings and make sure your OS isn't scheduling reboots. Make sure your power supply is rated for your cpu and not overloaded with devices. Check the tech boards for known issues for all of your components. If your cpu is under warranty and you can't find any existing known issues then replace the cpu - better safe than sorry.
And for Pete's sake make Microsoft earn their freaking money by hassling their tech support! You paid them - they owe you.
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