could be a partition issue, but i doubt it. Try installing again, make sure you format the partition before installing.
It sounds to me, that your Vista is not installed on C: If it was, then the line would work. C: is what i used, but if your computer happens to have a partition on it before Vista, Vista may be on D: or some other letter.
The only other option for this message is if, your vista bootloader is corrupt. Restart with the vista dvd and choose the repair system bootloader option instead of the prompt option. Once this is fixed, the method should work.