Open the "System Control" (Vista view, not classic view) and click on "Hardware and Sound", then select "Manage Audio Devices" (or something like that, I'm using the German version) under "Sound".
Now you (should) see a list of your speakers and/or headphones on the "Playback" tab, and your microphone on the next tab "Recording". Double-click on the mike symbol and select the "output" (or so) tab (the 2nd one, the tab in the middle).
Now you can set the output volume or level and the mike boost, up to +30dB.