Panic! Don't Panic?

One of those things I tell people will eventually happen to them happened to me tonight. I was editing a Word document, saved it, tried to quit Word, and it would not. The whole PC hung up with the HDD drive flashing frantically. I pressed reset. The PC tried to restart, the BIOS reported no HDD, then a Black Screen Of Death:

BOOT DISK ERROR - INSERT SYSTEM DISK AND PRESS ENTER

Gulp.

Found Bart PE Boot Disk, inserted, pressed enter. It booted into Knoppix. Started Command Shell, did C: - DISK NOT READY

Now wondering how long to rebuild this, and what I had on its C: drive. Shouldn't be too much apart from Music. Database I was working on backed up last night, phew.

Turned it off to think. Half an hour later, turned it on again and it started normally.

What do I do now? Turn it off again, never turn it off again, back everything up - how?

Still thinking, PC still running, 4 hours later.