Things will get seriously fracked up when two differnet operating systems try and access the same HDD partition at the same time.
How can this happen?
I found out.
1) Run some programs in windows xp.
2) Tell XP to hibernate
3) Boot into linux
4) access FAT filesystem, say to copy some music over to the d:\music directory.
5) reboot to windows.
6) Wait til it hits the fan.