One thing in my bag of tricks is a program called "Spinrite" I'm not a rep for them or make any money by this btw. Go to grc.com And get a copy. You just boot the CD (You may have to change boot order in BIOS so CD starts before anything else) If you hard disk has some read errors this is the most reliable way to get your computer working. This doesn't fix OS problems like scrambled directories but actual read errors. Always take care of this first. The guy who wrote this program knows hard disks like no one else and if this doesn't recover unreadable sectors? Nothing else will.
Assuming you get it going again (Spinrite has fixed a lot of computers) make a full backup (External drives are cheap)
BONUS: Running spinrite once in a while will give you a pretty reliable clue when the drive is starting to flake out.
The grc.com site has a lot of free goodies and some fairly advanced security tools. Check out "Shields Up" he is clearly a Star Trek fan.