From Control Panel, open "Sounds and Audio Devices Properties"
Click Advanced... button in "Device volume" section
Volume Control window should show up
From main menu, select Options | Properties
Select Recording radio button in "Adjust volume for" section
Make sure Microphone is selected in the list box at the bottom of the Properties dialog box. Click OK to close it.
Now the Volume Control window becomes Recording Control
Make sure "Select" checkbox is checked for Microphone, move the volume slide at the way to top.
If this doesn't help, select Advanced Controls from main menu Options.
Click Advanced button for Microphone volume setting and check the "MIC Boost" checkbox or AGC (auto gain control).
Depending on the sound card and driver, not all options mentioned above will be available. It's just a general guide to troubleshooting MIC problem.
Good luck.