Step One: Disable Auto Reboot On System Crash - http://www.theeldergeek.com/auto_reboot_on_system_crash.htm
Step Two: On next crash, write down printed content of error message, exactly as it appears onscreen.
Step Three: Post content here and someone will try to advise you concerning what is happening, why, and how to try to overcome it.
The other obvious possibility is overheating due to poor air circulation. Make sure that your system fans (1 for CPU, 1 for power supply, at the minimum) are spinning properly and check your system for dust/dirt accumulation.
Cleaning A Laptop Computer - http://mobileoffice.about.com/od/usingyour...cleanlaptop.htm
Cleaning the Interior of your PC - http://www.bleepingcomputer.com/tutorials/cleaning-the-inside-of-your-pc/
System manufacturer and model might be useful info.