Unplug teh CD drive internally leaving just the one Hard disk, GFx card, (take out sound card temporarily) memory(one stick only if you have more than one).
Try it at this point. If it works start adding things one by one till it breaks again. Then you have a clue as to what could be wrong. post teh results of your tests.
If it still hangs with just the bare minumum then you need to try a spare hard disk.
To do a low level sector check follow teh details in this article
How to perform disk error checking in Windows XP