It seems very weird and I can't understand it,
Why xesc, the output of Expression Encoder 4 Screen Capture, can't be play on Silverlight??
Why ScreenCaptureVideoProfile codec is always unknown?
doesn't the left hand know what the right hand is doing??
Answer from Microsoft Expression Encoder team
Being optimized for screen capture, the XESC codec has dependencies that are platform dependant and thus doesn't really fit in the Silverlight model.
http://social.expression.microsoft.com/Forums/en-US/encoder/thread/12115880-e529-477f-a44d-2577e30770ac/
