Windows Live Messenger Mic Problem

Hi, I hope someone will be able to help me with a problem I've been having.

I tried to hook up an Altec Lansing headset to my computer (Windows XP) since I don't have a microphone. I plugged both jacks (headphones and mic) into the proper ports at the back, but when I go to the Windows Live Messenger audio and video setup, my microphone doesnt register any sound. The headphones work perfectly, however. This headset was recently used a month or two ago, so I doubt it's a physical problem with the headset. As the setup suggested when no sound registers, I checked the volume of the mic by going Start > All Programs > Accessories > Entertainment > Volume Control, or by clicking the icon in the taskbar. I switched to volume for recording, ensured the microphone box is checked, and switched back to playback volume. Then I noticed that the microphone box for Mute was checked, so I unchecked it. A funny thing happens whenever I go to the AV setup in Windows Live Messenger, though; keeping the volume control window open so I can keep an eye on it, I go through the setup until I reach the microphone bit. The microphone box for Mute re-checks itself whenever I hit that stage in the setup, and unchecking it while I'm still in that stage doesn't change anything. Going back or forward a stage and unchecking the box and then returning to the mic setup stage doesnt help because the box still re-checks itself. Any ideas on how I can fix this and set up my mic?

Sorry to go into such a lengthy description, but I figured more detail would be better than less. Hope someone can help. Thanks.