A Live Linux and attempt to run it off the CD. If that fails to start you know that you have a hardware issue and then you will not be wasting time attempting to repair Windows when there is nothing wrong with it.
You can download for free one of the many from here
http://tinyurl.com/23nmq
You need to find out if you have a Software or Hardware Problem before starting attempting to fix the unit.
Worst case you can remove the HDD fit it to a USB Caddy and recover your data on a different computer.
But now is the time to Instigate a DR Plan to prevent Data Loss when your Computer fails. If you don't do this now it can cost up to 46K + to recover data off a Failed HDD. When compared to that a Few Hundred $ is nothing.
Col