In my case it was the USB-Driver of a USB Gaming Pad (c:\windows\system32\DRIVERS\xusb22.sys)
As soon as I deactivated the USB-Device in the device manager, the problem was gone.
If I were you, I would unplug all my USB-Devices except the cam. Then try, if Youcam works.
I'd also try if Youcam works in Windows Save mode.
I am so happy now.