Next time you start your machine, hopefulyl there will be no problems and you can use any program with the webcam in which case, you can delete the entry that is disabled. Next, use Microsoft / Sysinternals Autoruns and locate then disable (don't delete yet) the program from starting. Now, download Microsoft / Sysinternals Process Explorer and try doing a search against these files, it should be able to show you what file/process is hogging the camera. I would launch Device Manager again and go to the webcam, click the Drivers tab then the Driver Details button and make a note of all the files. If I was you, I would go to Device manager and simply disable then enable the webcam which should release it from whatever program is grabbing it.

There is no real easy way to know what current application is using the webcam, but when most applications loose the webcam, they do not automatically reconnect.

Any application that ties to a webcam gets exclusive control over it. The errors you are showing are pretty standard.

Manycam installer stopped installing