Ok so I suggest you do the following:
1) When you go into ventrilo make sure that the input and output device in the setup tab is set to whatever your mic is called.
2) Go into your control panel and then into sound settings and make sure that in the playback and recording tabs that your microphone is set as the device with the green check.
3) Update your computers sound drivers. To do so go into your control panel, then hardware and sound, then device manager and under audio click on each one and click update driver. Then hit search automatically and let it do its thing.
4) Look at your microphone and see if there are any loose parts or wires that are showing. If there is that is a contributing factor.
Finally, if you could tell me what kind of headset you have that would help as well. Hope this helps!