Have you gone to Control Panel, Sound - Recording Tab and see if your microphone is listed? If not, right-click in the window below the tab and check: "Show Disabled Devices" it will probably show up then.
IF you have done so already, download the latest Realtek HD Audio driver from either the link above that TorrentG gave you or from FileHippo which is a faster download link.
http://filehippo.com/download_realte...udio_vista_64/
Even though the driver name has Vista in it, it is the correct Windows 7 driver.