Try the following.
Double-click the speaker icon in the bottom-right hand corner of the screen to bring up the Volume Control menu.
Go to Options --> Properties and select the Playback button. Make sure "Microphone" underneath is one of the checked options.
Click "Mute" under microphone
Basically, the Microphone slider under Playback chooses how loud sounds going into the microphone play out of the speakers - the reason why you can hear yourself blowing into it, even when you're not using any other program. Muting that means that feedback ought to be minimised - unless another program itself is causing your voice to be played out of your own speakers. I can't for the life of me remember if Ventrilo did that, or had the option to.
Hope this helps, anyway.
(To actually change the input volume, do the same but instead go to the "Recording" button instead of "Playback", and adjust the Microphone slider accordingly.)
