IIRC bootable 'buntu flash drives also have memtest86 (you can make one with a boot CD)

windows 7 should have a simple memory test too. I know that vista does anyway. Run that first, if you find it didn't detect a problem then setup memtest and let it run for at least a few passes.

also if your video card fails, when you reboot, windows 7 will actually pop up at the bottom corner with
"Windows 7 encountered an error, click here for details" and it will say "Video card falire" or something. it's quite helpful

Well, the memory test turned out fine. This is weird. Is there anyway to test my PSU?
________
Dodge Town Panel Specifications

As you have an intermittant fault i would start from scratch.
Replace the PSU first as you have done a ram test and if it still fails after that you know its the ram, motherboard or graphics card.
Also check that the CPU is seated properly and even, with a new app of silicon, that means taking the CPU off the m/b and checking before re-install.
If your competent enough take your time and take the whole thing apart and start from scratch using proper antistatic precations and check / clean everything as you go, if not take it to an engineer.
You did not state how old the build is, the biggest problem is heat so bear that in mind whatever you do and look for any hot spots that have been created on the m/b etc.
Good luck.

It's an almost 3 year old rig. I could take it apart bit by bit. I practically built the system myself so I know every inch of it.
problem is, replacing the PSU is expensive and I don't want to throw more money on this rig since I'm saving up for a new one in the future.
Oh well, I don't have a choice do I?
________
Weed