If your microphone isn't working, try the following...
1. Go to "Control Panel".
2. Click on "Sound".
3. Click the "Recording" tab.
4. Click on the device you are trying to fix.
5. Now click the "Configure" button and then click "Set up Microphone".
6. Once that is complete, click the "Properties" button.
7. When you are done with your changes, in "Properties", click "Apply" and then "OK".
8. Then click "OK", again, on the main "Sound window".
If this doesn't work, it may well be that your jack either is output only and not a combo jack, or the combo input/output jack isn't functioning correctly. Both of these is really common.
In this case your best bet would be to get yourself a 3.5mm to USB adapter and connect the headset/mic...