Hi,
Go to control panel and click on sounds and audio devices or you can right click on sound icon in system tray and select adjust audio properties and see if your external Mic is showing up there ans is set as default. disable all other MIC's showing up there and click OK to save the selection. click start select run and type sndrec32 click OK. click on record button and record your voice for play back. Ensure that on your external mic mute button is on logic 0.
Good Luck.