First you need to make sure that the sound card is properly installed in your computer and its drivers are also installed. For this check, right click My Computer / Computer then Properties followed by Hardware tab and Device Manager. Make sure that under Sound category everything is running fine and the drivers are installed.
Proceed further and plug the microphone in the pink input on the back of your computer. Go to Start > Run and type sndrec32. Press Record and say something. The wave bar should display activity and you can stop the recording. Playback the content and if you can hear yourself then the microphone is working good. In other case, you might need to replace the microphone because it's possible that you have a faulty device.