ok i have a sennheiser PC150 headset with a pretty decent computer. its not USB (uses pink and green jacks) and whenever im talking to people over xfire or teamspeak, they say they hear a backround "white noise". i dont know what to do or how to fix it. if someone could please go step-by-step on how i would go about fixing this, your help would be greatly appreciated. (ive tried other non usb headsets and i get the same result)

Have you tried changing the various levels (including things like bass boost) on the mic?

Open your mixer (little speaker near the clock) and look over what options you have for the mic (both playback and record screens)...try things like making sure +20 dB boost is not ticked.