download memtest86 install it to a floppy, boot the system from floppy drive with floppy in, allow it to run- 7-9 tests, will take a few hours, if errors appear, either the ram is bad, ovcd, timings are too tight, or the CPU is overclocked.
My friend had this same problem . Well it turned out that his RAM is damaged. He had one 512 DDR2 RAM. The PC worked fine when it was started windows booted properly but after using the computer for bout 15 Minutes a blue screen appeared it said "Started Dump of Physical memory" first i thought there is some error in his HDD but after we took it to a hardware store we were told that RAM is damaged, We replaced it and now everythings fine.