Right click My Computer > Advanced > Startup and Recovery Setting > Disable Automatic Restart. This way if it's crashing due to a blue screen, we'll actually see it now and might get useful information from it.

Also, yes, if it's crashing/rebooting in such a random manner, doing a memory test to make sure the RAM's not faulty is a good idea. Memtest86 will do, and only needs a blank floppy/CDR/CDRW to run