I did a google search for removing the ad and did so and it worked! Problem solved. About 30 seconds after I'd done it, the ad on my other pc had changed to something different that didn't take over the sound card! I guess the ads change every 24 hours.
Oh well, thanks for pointing me in the right direction anyway. I used
http://www.lancelhoff.com/2007/05/25/remove-adver...ws-live-messenger/