I'm not 100% sure right off how to do this through Vista, but but in most any other copy of Windows...
Double click the speaker on the task bar to get Volume Control. If you don't see microphone, go to options and properties. At the bottom, check Microphone. Now back to the mixer, you should be able to control your mic's volume. This is NOT the recording volume, but the volume coming to your speakers. To control the recording volume, do the same steps, but select Recording from the properties menu.