How to reduce Microphone echo in a headset?

I've used two headsets, both Logitech, that have both worked on another computer.. but when I bring them to my computer, I run Windows XP Media Center 2005, and jump on Counter-Strike: Source, I get crazy echo in my headset. If I talk while I'm shooting it's chaotic, the sad thing is, when I unplug the headset from the headphone jack, and use my mic while using my computer's speakers, there's less echoing! I've never dealt with this before, I ume it has something to do with my computer, because I KNOW these headsets are just fine. Any help is appreciated.