Question how XP can repair itself

I was scanning a picture, and as the jpg was being written to the hard drive, the power went off. Later, when I tried to start the computer, I just got a screen something like "insert boot disk". I turned it off and on, but no change. Somebody looked at the computer and did a few tests on the boot screen and he said the hard drive was probably destroyed. A few hours later I tried to start the computer, this time the "check disk" came on. It seemed to finish the check disk normally, then Windows started. It seemed to boot up OK. I shut the computer down and restarted it and everything seemed OK.
My question is: How can Windows seemingly repair itself in this situation?