I always thought that the "c" disc was the partitioned part that controlled boot up and system control. It was traditionally on the faster moving portion of the outer disc and read first by the armature from start up. Over time that portion of the disc wears, usually first, due to heavy use. Replacement necessary perhaps?
In which case, and assuming you still the Windows CD, I'd suggest doing a scrub and reinstall and then restoring your latest backup.
When you get the option of where to install Windows, delete all existing partitions and let the installation routine create a single volume which it will format and install Windows into.
Also, as an aside, Vista is seriously out of support now. Might it be time to cut your losses and invest in a new machine with Windows 10?
Vista no longer supported with Security updates. You can get Windows 7 with an installation dvd for about £15. Windows 10 can be downloaded legally from Microsoft. Then purchase a key from ebay for same price. Both legal. I've done this many times without problems.