Hi Mark,
Is the pc a laptop or desktop and how many USB ports does it have?
A USB controller generally support 2 USB ports, if the ports are physically side by side then they will probably be using the same controller so depending on power requirements, it might be that the controller can't deliver enough power to both the dongle and the camera.
You can check the controller status under device manager (any exclamations or question marks show when you plug in both?)
You can also check that the USB ports aren't giving any I/O errors by using two low powered devices in the ports (e.g. keyboard & mouse).
An external powered hub can be bought for under a tenner these days so that's an option but first off all, check your device manager and see if there any issues there that show up.
Also ensure all your drivers are up to date, Windows Update has been ran etc.
Cheers
Al