Go to volume control and go to advanced properties and click on the radio button for "Recording" then check off Wave Mix Out or Stereo Mix out, something to that nature, it varies with what sound card you're using, whether it's a dedicated board or integrated board, but that's basically how it should work.
The settings should go over to Ventrilo, but if it doesn't just click on Setup in Ventrilo and where it says 'Hardware Input Mixer' select your sound card and the drop down menu for 'Line' should be the same as the option that you checked off earlier in the Advanced Volume Controls.