Run this:
MemTest: RAM reliability tester for Windows
Select as much ram as you can (but not more then there is fysical ram in the system - otherwise it'll start swapping to harddisk wich is slow and useless) and let it run for a long time - upto 1000% or so (not 100 ... 1000).
If there's anything wrong with your memory subsystem, this proggy will find it - it works way better then memtest86 wich has passed defective memory more then once over here.
If you get memory errors in this program, try removing 1 stick of ram, and then the other, also try using diffirent slots as the slot itself might be defective until you get no more errors.
If you don't get any errors with this program you can be fairly sure the memory is fine... then you can move on to test other things, but memory is the prime suspect with errors like these, besides malware ofcourse.