Some laptops don't have different connectors for microphone and line in. So, everytime you plug a microphone, system asks, in a popup: "Is this a microphone or line in?". Fortunately there's a checkbox to "do not ask again". Unfortunately, if you check it, the microphone doesn't work anymore, and you can't get that popup again (here there is an example; the only help was reinstall windows!!!).

The real solution is:
Control Panel → SigmaTel Audio → Advanced → Check "Jack reconfiguration popup" (or something like this).

Hope this be useful!

PS: this page helped me.