I've had this problem with multiple headsets--basically, the microphone picks up sounds from the headset. If the sound is loud enough to be picked up by the microphone(which is as far away as it can extend--it's built into the headset), it transmits as feedback--the louder the sound, the more painful the feedback. I'm convinced this is a problem with my onboard sound(either the quality of the hardware isn't good enough or the software is improperly configured). I'm using Realtek HD Audio drivers, up to date. As far as I can tell, i've only had this problem since I installed Windows 7.
There's also a consistent hiss sound when the microphone is transmitting(I test in Ventrilo by using two instances of myself, both transmitting and receiving the same sounds), though i've minimized it using the equalizer utility. This happens whether or not the microphone is plugged into the headset(it's detachable).
Thanks in advance to anyone who can help.