[PROBLEM]
VirtualDub says that the Video Capturing Device is busy (or that it cannot connect to the driver), using nVidia WDM Drivers.
"Error 418: Cannot connect to capture device."
[SOLUTION]
I've downloaded a very small program called "AMCap" (http://noeld.com/programs.asp?cat=video) and after starting/stopping it, VirtualDub worked fine. (strange, huh?)
Seems to be some kind of initialization which is NOT done by VirtualDub, but by AMCap.