Try running it with the case open/removed, see if you can figure our where the click & whir come from.

Download, unzip, install and run Everest Home Edition
http://www.comcen.com.au/~fed/everest.zip

Select Computer>Sensor from the left hand screen and check your temperatures.Indeed I have 2 Western Digital HDDs, a WD800JB-SE & a WD1800JB-SE. I have spent the day running CHKDSK on all my partitions. As I write this I am running Data Lifeguard Diagnostics (from WD WEBSITE) on both hdds, Extended test mode. All SMART tests were marked with PASS so I'm not sure the diagnostic will find anything.

As for the click SOUND, to me it sounds like a HDD 'parking' if you understand what I mean. The clicking sound still happens to me however the PC merely stalls for a few seconds before resuming function. This is small progress.

I took my PC case to a repair guy who agreed to take it home with him and try to fix it. He rang me later that night to tell me he could find nothing wrong with it, and had been happily playing games on it for a couple hours - he didn't charge me but when I took it home and plugged it in nothing had changed. First it loads xp fine, then I test it by starting up a game, then it crashes. The next few times I try to boot it up it freezes on the xp LOGIN screen.

I still can't explain why it worked fine for the repairman, and he is stumped too. I'll try it with the open case Fed. I assume its coming form the HDD bc the click and whir sound similar if not exactly the same as the sounds your computer makes when it starts up. I had recently found,though, that the screws holding the motherboard into the copper risers had a little washer thing in b/w the screw and MB but after removing it i thought i fixed it bc the computer ran well for many hours but eventually when it made the click, there was no whir, the computer automatically restarted and the click was even louder. (it sounded more like one of those metal toggle switches being flipped). and thanks for the software.

autechre: read the last parageraph i just wrote and see if that applies to you. CHKDSK is useless, and i got the same results from the data lifeguard software. what version of windows are you running and do u have any problems with USB devices?The temperatures seem fine. Here are the following things I found interesting/suspicious:

CPU Clock 2400.49 MHz (original: 2400 MHz)
CPU FSB 133.36 MHz (original: 133 MHz)

My Temps:

Motherboard 19 °C (66 °F)
CPU 18 °C (64 °F)
WDC WD2500JB-00GVA0 22 °C (72 °F)
Quote

I took my PC case to a repair guy who agreed to take it home with him and try to fix it. He rang me later that night to tell me he could find nothing wrong with it, and had been happily playing games on it for a couple hours - he didn't charge me but when I took it home and plugged it in nothing had changed.

I had recently found,though, that the screws holding the motherboard into the copper risers had a little washer thing in b/w the screw and MB but after removing it i thought i fixed it bc the computer ran well for many hours but eventually when it made the click, there was no whir, the computer automatically restarted and the click was even louder.

There seems to be a clue here, what was different at the repair guy's place?
Different electricity outlet?
Using a different power strip or none at all?
Different keyboard, mouse, monitor or any other extras?
Can you try and eliminate these things as possibilities, I'd start with the electricty outlet & power strip.