I have being engaged in PC troubleshooting for years now, and the main lesson I learned during these years is this: in order to successfully find and eliminate a problem your acts have to be successive and follow inductive logic, i.e. from the simple to the complex. In other words, you should, first, take out of the way all the simplest possibilities and only then test ones that are more intricate.

As to your problem, Ainol, if I were you, I would begin checking the mechanical issues of the USB connection (USB receptacles, the cable, etc.). Then, I would install the CMU on other PC and test whether that PC could see the camcorder. And if the second computer would fail too, I would start pondering what could be wrong with the NX5 itself (firmware updates?).