This happens for one of two reasons.
Hopefully it’s just a settings issue, which is an easy fix. You can go to your Volume Control, click Options, click Properties and press the button to Adjust Volume for Recording so that you can change the volume of your microphone.
Unfortunately, it could also mean that your sound card could be bleeding the output into the input. You can tune this with the Inbound and Outbound sliders in Ventrilo to reduce it, but the only way to really fix it is to get a new sound card (which Guild Launch does not sell).