This is really wierd..
I tried the camera with Messenger and it worked!
Creative still said the camera was being used, so I uninstalled Messenger. Seemed obvious to me that the culprit must be Messenger, but no dice! Creative still wouldn't use the camera.
I uninstalled Camtasia, and this didn't help. Creative still claimed the cam was being used by another application.
I uninstalled Creative. I reinstalled it, and the camera works!
I reinstalled Messenger, and the camera still works!
I reinstalled Camtasia, and the camera still works!

I can only guess that Creative developed some kind of bug, because now all 3 programs are very happily living together without problems.
Thankyou very much for your assistance, and I am sorry to have wasted your time.
