Are you sure you have reset all overclocks to factory settings? Double check, maybe using CPU-Z.
If you are lucky windows detects errors and restarts automatically to prevent damage to your hardware. You can disable this in System and Security: go to system properties-> the advanced tab. Click Startup and recovery and uncheck 'Automatically Restart'. If it stops happening all together you may be good, or something may still be up. If this sorta fixes your problem you may decide to leave it as is.
Mind you, there is still something (slightly) wrong, you are just electing to ignore it.
You had the idea to post the event log, so you probably checked the obvious: is your pc filled to the brim with dust? Is there a FAN not spinning on your GPU? Check your temps, are they running high? Your BIOS also may have a setting to auto-reboot if temperatures get to high.
I've had experience with restarts after booting up games, and sadly there are many things that can go awry: one of the memory dimms/slots may be bad, it could be the videocard being disconnected from the slot, or likely an issue with the fans on gpu or dusty cpu coolers. (edit: one time it was a bad case-fan profile)
Trial and error, I'm afraid.
Keep us posted