If you have a lot of things worth saving I'd get a new HD and image the old one onto it.
Most new HD's come with software to do that.
I just did it with a new Seagate. It now sits in its original box on the floor next to my desk with all my stuff backed up to it.
Once its backed up just try the new HD in the machine without the old one being hooked up.
HD's are fairly cheep now days, at least around here, and having a spare to backup to is a good practice anyway, if it's not the actual problem.
My last HD failure game me the blue screen before it died.
I'd suspect if your getting no warnings, and only a hard crash as if loosing power, then that's whats your experiencing.
It could be as simple as a bad connection, to a PSU or even a faulty Mobo