To turn off the sound on a ThinkPad running Windows XP:
Adjust the system volume ->
In the Device volume area in the middle of the Volume tab, click on Advanced. I know, I know, it looks like you are adjusting speaker volume, but go ahead and do it.
If the microphone doesn’t show up as one of the devices, select the menu item Options->Properties and put a check in the Microphone box.
In the microphone sub-panel, put a check in the Mute box.
(I am soooo not a Windows person…)