In my case, I went to the mixer tab and found that not only was my headset set as a microphone - so was my internal mic in my laptop and the mic on my webcam.
In my case going to the control panel and clicking on sound did not bring up the mixer. I had to go to the taskbar, click on the speaker icon which then did bring up the mixer. Then I was able to turn of the two other microphones the for some reason were now on all the time.
Once I turned both of those off, the feedback stopped. Why this happened all of a sudden after using this laptop for 4 years (its a Lenovoe X220 with Windows 7)) I have no idea.
Hope that helps