I'd test to see if the PC is overheating. Download some software to show you what temperature you are running at. I use Everest home edition - http://www.softpedia.com/get/System/Sys ... tion.shtml
Your BIOS has a setting to switch the PC off if it overheats - it is normally set to about 85-95C. The PC would normally run at 30-45C depending what it is doing. If you are much over that, clear dust out from the processor heat sink. It has lots of fine fins on it, and the fan sucks air through it. An ideal arrangement to trap loads of fluff and stop heat dissipation!
Switch off, take the power lead out and take the cover off. Let it cool down. Then, using a can of compressed air (eg http://www.maplin.co.uk/Module.aspx?Mod ... an&doy=3m4
) to blow the dust away. Put it back together and check the temperature.
If it isn't overheating, I'd then suspect a RAM problem. Check your RAM with an application like these:-http://www.memtest.org/http://oca.microsoft.com/en/windiag.asp
If you have more than one RAM DIMM, you can also remove them one at a time, to see if the problem goes away.
Finally, as a last straw, I'd find the manufacturer of my hard disk, go to their website and download their disk diagnosis tools. Run them, and see if they tell you anything.
It is worth downloading the memory test and disk diagnosis tools anyway and burning them onto a bootable disk. Personally, I think every PC should get a set when they are sold. You never know when you might need it, and when you need it, you probably can't download it!