First, try and mount the HDD as a secondary drive onto another PC. Run some diagnostic tests on it and make sure the HDD is not on it's way out. It is usually the first thing to quit in a relatively old PC. If it is tired, then best bet would be to do a backup of your data and get a new HDD.
If it is still good, you could d/l killdisk or something similar, make a bootable CD with it - it tells you how on the website - then boot that and zero the C partition. After that, try re-installing on C.
Keep in mind, if your PC still boots on C now, that will not be available after you do the zeroing.