Right, you may have to set your sound card to record the mic.
Go into the Volume Panel in the taskbar or by going to Control Panel, and first of all check that the MIC isn't muted.
SET TO PLAYBACK:
If its not listed, go to Options, Properties.
Make sure Playback is selected and tick Mic in the list. Press okay.
Now make sure its not muted, and put the volume up.
You may find on some sound cards there is an option for Advanced controls (Options, Advanced) which may allow a mic boost which improves cheap microphones.
SET TO RECORD:
Now, go back to Options, and go to Properties again, and this time select Recording, and check Mic on the list. Press okay.
Now, make sure that it is ticked this time, to be used as the recording device.
Your then done.
I hope this helps