1. Sound card; download the burn in diagnostic tool from the downloads section of the site, it's a 30 day trial, let it run it's diagnostic and it will tell you if you have a bad sound card or not.
2. Check your settings and make sure your computer is recognizing the microphone. Go into the sounds settings, through control panel, and go into the sound properties, and there should be an input tab, in that input section you should have the ability to test the input device.
If that doesn't work go into your device manager and make sure the drivers are up to date, if needed, uninstall and reinstall the drivers.
3. Your microphone is bad. Try plugging a different microphone into the computer, if it sounds fine, it's the mic, if it's still messed up, it's the computer, then refer to my first two solutions.