Windows 7 line in problem

I recently installed windows 7 ultimate and i can't get the mic line in to work. its the weardest thing. i plug in the mike to the mick jack and i adjust the settings to line in default but the mike doesn't register on the meter. but if i plug my headphones into the mike jack and speak into the headphones it works. at first i thought my mike was bad. i've tried three different sets and the same thing, the headphones in the mike jack works but the mike in the mike jack doesn't. weard. has anyone heard of such a thing. i know it sounds crazy, can anyone help?