If your computer is frequently self-rebooting during intensive operations, they are most likely caused by hardware problems. There are several possible causes.
First, your PC may be overheating owing to dust accumulation. Over time, dust can gather inside a PC chassis, layering over heat sink surfaces, memory modules, and circuit boards. You can blow the dust out with canned air. Open the case and check to see where dust has accumulated. Be sure to use short bursts, and get inside the surface of cooling fans mounted on CPU or GPU heat sinks.
Another possible cause is power-supply problems. For example, if you've recently added a high-performance graphics card, you may have exceeded the power supply's capacity to deliver adequate power. Or it may simply be that the power supply is failing.
Another common problem is memory that's going bad. You can easily test for this by downloading a free copy of Memtest86+ (www.memtest.org). Run Memtest from a bootable floppy disk or CD. It will run a series of tests on your memory to see if it's failing.