The Mysterious Moving Drive

I have an HP Vetra PC with W2000 loaded. Last night the C: drive (boot partition) moved to d: drive (not sure how or why at this point) and now the PC will not load past the login screen. If i do a CTRL ALT Del I can start the task manager and geta dos prompt or load MMC. I have tried to change the drive letter back to C: in disk management but since it is a boot partion it will not allow me to do this. Any suggestions.