In device manager you can disable a device but the next time if boots it will fnd the device. This is a good thing that wasn't possible before USB. USB gave the capability to uses devices without having to reboot.
I would think you wouldn't have to disable the built in web cam because it's software controls it and the Logitech software controlls the logitech web cam.
The only way I know how to use my Logictech external web cam with the Aspire 5742 is to go into Program Manager to disable the built in webcam.
Is there an easier way please?