When used by a man to a woman "You're acting like a cunt" (a really nasty bitch
only allot worse!) Generally, a man will get slapped by the woman, or be sleeping alone on the floor for about a week, or both.
When used man to man "I can't believe you did that, you really are a fucking cunt!" Now the man has not only insulted the other man in the strongest possible term, but he has also called in to question his manhood. A fist fight at this point is very possible.
When used man to man about a woman "Stay away from her, she's a real cunt." Generally a big THANK YOU is in order, along with buying the guy a beer.
Woman to woman, good question. I'm a guy so I can't say, and it is rarely if ever used that way. They don't seem to use the " C " word much.