Are you using the sound chip in your motherboard, or a separate soundcard?.
If you are using a separate soundcard, read the instruction manual and
follow it.
If you are using the motherboard soundchip, right click the speaker icon on
the taskbar. Click 'open volume control'.
If a slider for microphone isn't listed, click Options-Properties-Recording.
Put a check in the Microphone box, click OK.
A slider labeled Microphone should now be displayed, click Select, plug a
microphone into the microphone jack on the motherboard, and launch Windows
Sound Recorder. Start-Programs-Accessories-Entertainment-Sound Recorder.
Adjust volume using the slider.
If no sound is recorded, you may need to activate the onboard sound in BIOS,
to do that check the motherboard manual BIOS instructions, or contact the
computer manufacturer tech support.