Without knowing what sort of sound system you have, it's hard to give a good answer.
Check in the settings for your sound device (for example, VIA HD audio deck, not just "Sound" in control panel) to see if there are settings for which sounds go to which speakers.
Check if there is a setting to automatically disable the internal speakers when the headphones are plugged in.
BTW, is your Headset USB? If so, that could be the problem. You would need to set the sound controls to send all sounds to the USB device. I know that in Ventrilo it is possible to select from various output devices independently of the main sound. (Or you could get a headset that uses the normal speaker/mic jacks.) I don't know if it's possible to get the system to auto switch to a USB headset.
Make sure your sound system is set to "stereo" or "2.1".
It seems to me there was another post about this recently.....
Last edited by Quaker; Jun 12, 2011 at 09:39 PM // 21:39..