This probably applies to any MTP sync'd device, but I'm specifically referring to the dmpj-210 512MB Napster player. It worked fine with Vista for years but suddenly was not recognized by the computer, and any attempts to reinstall the driver ended with a "code 10" error. If you've installed libusb-win32 for any reason (like, I don't know, to jailbreak an iPhone or iPod touch) then the problem is with the file libusb0.sys. Uninstalling libusb-win32 should restore Napster-to-go.
Here are the solutions I find on my own once customer support tells me they've exhausted all options.