Another Windows XP tip, from the dark corners of my own personal experience.

The problem: You turn on your computer, but Windows doesn’t start. All you get is a blue screen that says UNMOUNTABLE_BOOT_VOLUME. What, oh, what do you do?

The solution: Boot into the Recovery Console. For this you need your XP CD, and go through the motions like you’re going to reinstall Windows, but look for the Recovery Console option instead. The Recovery Console is a command-line interface with diagnostic and repair tools. Once you’re in the console, enter these three commands in turn:

chkdsk /r
chkdsk /p
fixboot

The first one can take half an hour or longer, the others zip by really quick. After that, reboot your computer, and you should be golden!