How old is your hard drive, and what brand is it? What computer brand and model do you have? You likely will have to considered a total reformat and reinstall? But I would buy or obtain another drive so that you can connect to your current one as a slave or usb drive to drag and drop, or recover all your data. Sometimes that is necessary to make a corrupted install behave.
One of the usual problems is a failed hard drive... where the leading edge of one of the plates begins to bubble and flake off... but often they are drives with lots of miles on them... or exposed to a lot of heat. If a failing hard drive is the case, you will find the problem becoming progressively worse until you lose everything.
Finally, if you run a "repair" with an OEM drive, or a recovery disc from a computer manufacturer, an academic WXPH version, or an upgrade version, the limited repair it can do often doesn't work properly or at all. You need a full version that is not one of those I just listed, in order to have your best luck.