I don't know about the sd100, but I did it with my A75.
Procedure:
Download Cam4You Utilities, install (Some versions are for specific models)
Download
Camtasia Studio, install
Edit: Should say, first, connect your camera in review mode. Then, Start Cam4You, click Pictures-Take Picture Remote. Start Remote Process. Start viewfinder. Start camtasia studio Recorder. In options, set output to live. Click the red button. Center frame on the viewfinder in Cam4You. Boom, you got a webcam that yahoo, msn recognize
