XP demolished its own MBR on me last night. Fortunately, I had GRUB installedon my ubuntu disk, so I was still able to boot after a little tinkering. I am convinced this is part of MS's evil plot to make me run out of activations, thereby forcing me to give them more money.
I've seen that happen a couple times. Every time, it happened while I was playing WoW, and I got a bsod from my NVidia driver.
Here was a way to fix it:
http://forum.valhallalegends.com/index.php?topic=8752.0
Hah, looks like it did help someone down the line.