A few questions to ask to cut out possible solution:
What games are you playing that triggers the reboot (online games by any chance)? If you use a USB modem and the reboots happen on online games only or when you are playing games with downloads taking place in the background, it is likely the USB modem having conflicts with your PC (common problem in the past).
If it is a BSOD and your system is set to reboot automatically, it is possible that you are missing the error (your monitor might take a while to change to a different screen res/refresh rate when a BSOD is triggered, hence you could miss it). To disable automatic reboot:
-Control Panel
-System
-"Advanced" tab
-Under "Startup and Recovery", click on "Settings"
-Uncheck "Automatically Restart" under "System failure
Any BSODs that take place now will be displayed nice and clearly now.
Are you overclocking or using manual settings for e.g. RAM? If yes, revert to non-overclocked state and for manual settings, select "Auto" in system BIOS and let the motherboard decide what it feels is the correct settings. If the system is stable afterwards, your manual settings will need to be looked at.
Possible problems include driver conflict or buggy drivers, try different drivers?
And as others have suggested, PSU.