How to repair RtcDll to get Remote Assistance to work?

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I and another person are running Live Messenger 8.5. If the other person tries to request remote assistance, I can accept but I don't get to see his desktop. This used to work months ago, but I can't pinpoint what software I might have installed since then. There are several.

Checking event viewer, I see there are several DCOM events and text indicating that a referenced assembly is missing from RtcDll.

How can I fix this?
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