Due to some really strange disk partitioning I have on my laptop, I ended up in a situation where one operating system had an incorrect system or boot drive letter. Of course, through the Logical Disk Administrator in computer management, you get an error along the lines of "You cannot modify the drive letter of the system or boot volume". However, there's a useful knowledge base article which did the trick for me - Q223188 which applies to Windows 2000 and Windows XP walks you through modifying the registry to altering the system or boot drive letter. Nice.