Try START|CONTROL PANEL|SOUNDS AND AUDIO DEVICES
Go to the VOICE tab
Under VOICE RECORDING choose VOLUME
This will bring up the RECORDING CONTROL window.
Go to OPTION in the menu, make sure that in the PROPERTIES you have the correct device selected (which you most likely do), then below where it says, "SHOW THE VOLUME CONTROLS", make sure the MIC is checked.
Also in the OPTIONS menu, make sure ADVANCED CONTROLS is checked.
Under MIC VOLUME click ADVANCED and click MIC BOOST at the bottom and click OK.
Also make ure to increase the MIC volume, which would be better described as "sensitivity".
Here is a screen shot of another option: