Push to Talk
Try pluging your microphone into your line in rather than mic socket. Alternatively, reinstall your audio drivers.
This is usually down to audio drivers or a dodgy mic port.
The way ive got it to do what you want is, at least in Vista and Win 7, go to sound - playback - properties - levels and mute the mic..trust me on this - Go into recording, same as before, check that the mic is UN muted. Still able to talk with none of the feedback created, unless of course your running speakers then it still happens.