Then, very briefly, you can download an operating system that runs off of a CD. You don't even need ANY hard drive in your PC for it to work, just the CD. If your PC runs fine on that then you know that your motherboard etc. are fine and that the problem you have is either a corrupt registry/software problem or a hardware problem with your hard drive.
This downloadable OS (right click and save as) is called
SLAX KillBill Edition v 5.1.8. This is an iso file which you'll need to create a CD from.
This site gives you a few commands that you'll need to type in before you're using a familiar point & click/menu navigation.
You'll get onscreen instructions for most things, the only one you won't see (without reading that geocities site) is when logging on (
boot type : ) is to type :
slax copy2ram
(which means that once you're up and running you can take the CD out in case you need to play music, burn CDs).
This doesn't affect your existing Windows installation in any way. You'll also (nearly forgot) have to make a change in BIOS to tell your PC to boot from the CD. You could run this linux disc (and it's far easier than I ever imagined - it does all the configuring for you) and once you've tested with that, take the cd out, reboot and you'll be back to Windows.