What's wrong with my PC?

[hotelmariofan]

I have a Dell PC with the following specs:
OS: Windows Vista
RAM: 6GB
CPU: Q8200 Quad core @ 2.33Ghz
GPU: 9800GT (1GB)

For quite a while now the PC will randomly crash, or atleast that's what I think is happening. The screen will go black and will not turn back on, unless I press the power button on the PC (I have to hold it down) and then start it up, and click start windows normally. I'm assuming it crashed since it brings me to that screen. My question is, what part has gone bad? Clearly some part is bad, and I knows it's not the GPU as I have swapper the 9800GT out with other one and had the same problem. The latest time that it crashed, instead of the screen going black, the computer started making a very loud kind of repeating sound (I was playing music) it was like a loud grinding sound, not sure how else to describe it. Now I replaced the little battery on the MOBO and swapped GPUs, so I know that's not it. My next guess would be a failing hard drive. What are your opinions? Thanks in advanced.

 

[crewton]

One of your RAM sticks is going bad. I'd use only one stick at a time and run memtest 86+ and see which one is giving you errors.