Is my harddrive of Windows blowing up?

I wouldn't run Scandisk or defrag on a hard drive that's suffering from mechanical or electrical breakdown. If the drive is corrupting itself due to mechanical or electrical problems, those services will ensure that as much data is destroyed as possible. :\

Slowdown and occasional failure of the drive to initialize is a common sign of drive firmware errors...back your stuff up, and try to put as little stress on the drive as possible. Normally, I'd recommend that you not even start the thing up, but if you don't have a current backup and don't want to pay up for data recovery, you probably don't have much choice in the matter. Once you do finish backing the drive up, you can try a fixmbr command from the Windows recovery shell, though I don't think that it'll help.

Of course, the problem could be with the drive controller, or something else entirely, but my first inclination is that the drive's firmware is munged.

What model drive is it, BTW?

Jam it back in, in the dark.