Dimension 8400 and Webcam

I have an older dimension 8400 with XP SP2 and all updates.  Computer works great, but I cannot get it to recognize a webcam as a webcam.  Have tried two different cameras, different manufacturers.  In both cases the webcam will install and work locally, but is not recognized as a video input device for AIM, YAHOO or Windows messenger.  When I boot the computer I get a message "input not supported", which I presume refers to the camera.  Ideas?

Chuck