You mean one of the systems boots as something other than C ?
Have you lately reinstalled one ?
Windows (from Vista on) always installs as C, regardless of what pre-exists, providing you install it from the booted DVD.
If you don't boot the DVD, i.e. if you merely exec setup from the DVD from a running version of another Windows, then the running Windows (being C itself) must assign another (the first free) letter to the new OS.
If that's your situation, it's permanent unless you do the install again.
There's no problem running an OS like that, though you can occasionally run into some problems with older software which hard-codes C without checking.