Actually, from the workshops I've attended, having a professor from a class you got a C in write you letter could be a potentially greeat thing. But that would only be true if the guy knew you.
The two classic examples of letters are those that say "This student dominated the class and needed no assistance whatsoever, passed with flying colors, and is an ace. Accept him now because he'll kick ass." And the ones that say, "This student worked incredibly hard and showed extreme initiative. He came to me every single day and did an insane amount of practice problems and showed a great passion in his work. Accept him now because he'll work his ass off." And getting a C in a class when you did everything you could to succeed fits into the latter category.
But if the guy has no idea what you did or who you are, no matter what grade you got it wouldn't make for a good letter, even if you were ranked First in the class.