Computer can't boot

Hi, I have a problem with my desktop computer, it was working fine an hour ago before I tried to add another 256 DDR Ram stick to it.
At first I thought it was a faulty RAM, but when I took it off, the computer still can not boot.
I have also tried to tighten all other components (IDE cables, video card, etc) and reset CMOS, and it went to boot, but could not go in to the operating system (blank screen after the windows logo). Sometime it will only boot until the IDE scanning, and it stopped there for a long time.
So I need an input from you guys, which component do you guys think is faulty? I've tried replacing some parts with the working ones, like RAM, Video card.
I was thinking maybe the PSU has problem, but is that possible since sometime the system can go up to the windows booting screen..?
the PSU is 400W and has been running for 4 months or so.
Thanks guys, and I appreciate any input you can feed me :)
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CallandorConnect With a Mentor Commented:
Here are some things that could cause the symptoms you describe:

-IDE controller faulty (switch the cable to the other channel)
-bad IDE cale (swap cable with another)
-bad drive connected to IDE controller (run disk diagnostic for your drive)
-CMOS lost settings (reset or change battery)
-faulty or inadequate power supply (swap with one that is at least as powerful)

Disk diagnostics (choose the one for your drive):

Western Digital 
IBM and Hitachi 

CHiKuBieAuthor Commented:
I guess the problem does not come from my IDE drives, because i tried to replace/remove any IDE device and still having the same problem.
Any chance that the motherboard might be faulty? or maybe the VGA card?
A bad video card, I think, would hang your system completely, and you did say you swapped it out with one that works, right?.  You may want to start from square one with a bare bones configuration and add to it as you see things work.  Start with just the video card, one stick of RAM, the cpu, and no other devices connected.  See if it boots up, then try it with a bootable floppy, and then the hard disk.
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kunalnanda69Connect With a Mentor Commented:
It is also possible that the operating system itself has gone corrupt. can you give us the complete specs of your computer's hardware and software so that a detailed diagnosis can be done. I would suggest that try a complete restore of the operating system. Also remove any USB devices connected to the computer at startup.

Another thing that I would recommend is to go into CMOS setup and verify if the computer is infact detecting the hard disk.
RushBConnect With a Mentor Commented:
Simplest first - you said you had added some memory. Did you bump anything while you were in there? Did you try to boot, and make it part of the way into the OS? Have you tried making a memory diagnostics cd on a buddy's machine, or some kind of boot CD. Even boot off your OS CD and see if it will run.

Second thing to check - is this the correct memory for your machine? Did you check and double check that the memory is compatible with your motherboard, and the memory you already have? Same speed, ECC, non ECC??? Read your mb manual and see if what order it wants it in the slots? I have seen some three slot boards that want you to use one and three if you only have two DIMM's.

If all you did was try to insert some memory, then I really think you are looking at a reinstall after you check your system out. You may find that the memory is not compatible and it corrupted your install.

CHiKuBieAuthor Commented:
Turned out the CPU was faulty, I don't know why the system can still boot up without any warning sometime (system always hung at random time before finish loading windows). I swap the CPU with other similar computer, and is back to normal now. I would like to thank you guys with the support, those answers you've provided could potentially be the solutions. Cheers!
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