No problems, whatever you decide. There's certainly no harm in formatting if that's easy enough for you, but all the evidence is pointing to hardware.

If you do decide to use an external drive to copy your files, you can only really do this for your personal files, music, documents etc. Most apps will install files into other area's too, such as your common files folders, and windows folders. Some also create registry settings during installation, which are impossible to copy individually. You'll need the original installation files or installation CD's.

Good thinking Holly

I would be surprised if it was the direct cause of your disk problems, but then this kind of software does put a lot of stress on the PC > constant disk activity. Some people experience loss of internet connectivity, but it is also a breeding ground for nasties/viruses, and generally used to download illegal content in any case.

I wonder if this shop did any kind of stress tests on the PC?

No prob's. There are a couple of other software tests you can do yourself first, using software such as PassMark or HeavyLoad. These are only trials, but may give you the opportunity to reproduce the problem.

Also, if you download Speedfan, there is a tab called SMART, which will give you an idea of the actual health of the hard drive (assuming the HDD supports it). < not sure why I didn't mentioned that one previously.

wow Holly, I bet you were not expecting to find graphics card issues!! Although the real question is, do you experience any graphical issues and glitches at all? I mentioned that the temps were slightly higher than usual previously, but Speedfan is fairly generic in terms of what it considers overheating (flame icon). This is because there are different levels of temperature threshold between components. Speedfan is very dumb when it comes to recognising these thresholds. The NVidia cards can handle temperature up to (and sometimes exceeding) 100c when at 100% load for an extensive periods of time (playing 3D games for example). Preferably below 100c of course, so you would expect to see idle temperatures below 60c. Yours is 62c, which isn't perfect, but not going to cause problems at that temperature. If your not much of a gamer, there's absolutely no need to worry. If you are, you may wish to consider additional cooling before this becomes a problem.

Your real issue at the moment appears to be some issue between the motherboard, hdd controller, cable, or hard drive.. most likely the hard drive itself, but seems to be impervious to any kind of testing!!

You detected no HDD related issues during your stress tests? It's seems quite old by all accounts, it has no SMART technology, no temperature sensor. It is also the most likely candidate, as with all moving components, it is only a matter of time before they do fail. So this might be a possible next step you might want to consider... perhaps spend a bit of cash, replace the disk and add some additional cooling at the same time. The only issue is that we can not do any further testing on the disk to be absolutely certain.

Yep, you would still need extra cooling, as these cards can pump out quite a bit of heat. You may also need to check out your existing power supply to make sure it has enough juice to support a 9800gtx.

As for software, I would have thought any kind of stress test would have been enough to reproduce the problem.. as you mentioned, it took your CPU (for example) to 100% load.. if that isn't stress.. I don't know what is!