it is also possibly a failing hdd or power supply. also, it could be the ram or the mobo.
-look at the mobo for any bulgy, leaky capacitors.
-first off, test the ram with memtest, might be included with a linux cd.
-then, if possible, test the psu with a psu tester. and monitor voltages with speedfan.
-then, check ur hdd for bad sectors with super f disk