There are 2 known reasons for this:
- Your sound card is "bleeding" it's output channel into the input channel. To help reduce the echo, you can tune your Inbound and Outbound sliders within Ventrilo. This probably won't eliminate it, but will help reduce the loudness of the echo. The only way to truly fix this situation is to buy a new sound card.
- Your Windows Settings need to be adjusted. Your microphone volume was activated in your "playback" volume in Windows and not your "recording" volume. Go into your Windows Volume Control, and mute the microphone on that window because it needs to be. Then, in the Volume Control window, go to Options, Properties, and press the button for adjust volume for recording. Then turn your microphone volume UP there. Also have the button below microphone that says select checked.