Never had a problem like this but my understanding of how it happens is something along the lines of:
People see drives during Windows 7 install, click around, decide what to do with them then ASSUME that the top drive (normally C) is the one Windows 7 will install onto, where as it's actually the currently selected (not always highlighted) drive that it will dump the OS on.
4 of my mates have done this so far, it's ended up on D E and F drives, when they all thought they had C selected. Been a bit of a pain to strip out the old OS install from their various torrents/garbage etc scattered in the root of D/E/F too