are you completely sure that the jack is actually a combo one, input/output? As many computers anymore have a single jack that is output only. If that is the case, that would be why your microphone isn't working. However if you are certain that the jack is input/output then you can 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 help, then you may want to test...