I bought a new Zen

I bought a new Zen Vision M. I connected it to my pc after charging the battery, and I managed to upload approx 250 mp3 files before the process stopped. Whenever I connect i to the pc now, I get an error message saying "unrecognised device", and am no longer able to administer the tracks on the device. The display shows "docked", but I cannot then do anything without resetting the device, and I still cannot connect to the pc. Any ideas? TIA.