First make sure that the 3.5mm port you are connecting to is actually a dual, input/output one. If it isn't, then the mic will never work in it.
Next 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".
Should this not help either, then I would suggest getting a 3.5mm to USB adapter and connect through that.