Ooh, I've gotten that.
Though you can sometimes repair that error either with a WindowsXP CD and accessing "Recovery Console", just to be safe, if you have another computer, mount it to a USB enclosure and see if you can access it that way, and then back up the files.
The unmountable boot volume is a file system error. I'm not sure what causes it. Could be a bad 40-pin old ribbon cable, or it can be Windows (or a virus) messing up the file system table. There's a small chance that if you access the drive such that it's not the Windows main drive, but rather as a secondary drive (like how I said before, with an usb enclosure on another computer) that the error will either not manifest, or you will have some rudimentary file access.
Back up your stuff! z0mg!