You need to change the microphone settings in windows, put it louder there and click on mute. The feedback you experience is a local sound card setting as i understand.
There are many ways to access the settings, one is via the configuration settings of your sound system. clicking on the little loud speaker in the windows task bar. You have to tick some switches there to find the microphone lvl, diffrent for each sound card. mostly options select the line in to show up.