Heat is probably the culprit if it shuts down unexpectedly but if it REBOOTS unexpectedly then it is likely a Windows problem. If it is rebooting then try disabling the automatic reboot on error and hopefully it will just throw you an error instead of critical failure.
To disable automatic REBOOT:
1.Right-click My Computer, and then click Properties.
2.Click the Advanced tab.
3.Under Startup and Recovery, click Settings to open the Startup and Recovery dialog box.
4.Clear the Automatically restart check box, and click OK the necessary number of times.
5.Restart your computer for the settings to take effect.
If it is a heat issue then used compressed air to clean inside the case, fans and heatsinks. Inspect fans while running to ensure they are spinning fast, replace as needed. Fan are cheap ~$10-15 bucks each.
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