Hmm I see. You'll have to forgive my presumption that your initial running of the WD Diag' tool was in DOS. I didn't think to consider you were running it in Windows, I thought it would have been a DOS specific program.
If I had realised I would have been a little more sceptical about the cause of your OS crashes, nevertheless... Was the second succesfull attempt at testing your HD performed in DOS or Windows?
I think it would be pretty safe to say that if the hard drive managed to make it through an extended test without errors, I can now summize that it's probably fine. So you can hold off on the shallow grave, at least for the moment. Quick question though, when you performed a re-install of the OS, did you format the hard drive? if you did, quick or full? Is there a format function in the WD diag tool?
You've got the right idea though, by swapping your components around one by one, and might I suggest, if you have more than one memory stick you reduce the amount (for testing sake) to just one stick, and perhaps try moving it's DIMM slot position as well. Also make sure all your BIOS CPU, memory and BUS timings are all pretty laid back, whilst at the same time checking again that everything is as it should... e.g AGP is Init Display First etc etc.
How are your voltages looking? Got a spare PSU you can swap your current one with?
Could you also describe the freezing you are getting in a little more detail please, is the freezing followed by a BSOD and/or restart? do you get some funny lines appearing accross the screen?
I however am still slightly confused about what is going on, simply because of the below quote. You indicate that you tried the HD in another system, and it still had trouble getting it to work? Fancy trying that again? You see the problem is, you now say that the HD is question has passed an extended manufacuturers test, which indicates it is fine, but the fact that it wouldn't work in your newer system indicates that there is still a problem with the drive. What OS's are the Gateway and the newer PC running? what file system is or was the HD? You also said you had trouble finding the HD in the newer systems OS? did you check in Disk Manager? (Disk manager being Windows onboard formatting and partioning tool)
So after the HD wouldn't boot the old gateway computer i put the HD into my new computer as a slave (A7N8X-DX Mobo) and the computer booted into windows fine and then said it had found the HD and installed it. So i checked my computer and the HD wasn't there, i restarted and it still wasn't there. Then i switched the old HD to the master and my HD to the slave and tried to turn the computer on but the computer froze after that colored ASUS boot screen every time (i tried 3 times).