If you plan to use an external microphone a good thing to do is to set that microphone as your default recoding device.
To do this go to the Start Menu and click Control Panel. Once the window opens click on Hardware and Sound and under Sound click Manage Audio Devices. This will open up a new window click on the Recording tab. Now the window will have a list of microphones that are attached to your computer. What ever one has a check mark on it is your defualt microphone. Click on the microphone you want to use and click Set Default. Finally click OK and the microphone is ready to use.