I've recently started getting the following error when trying to sync from WMP11

Your device requires that this file be converted in order to play on the device. However, the device either does not support playing audio, or Windows Media Player cannot convert the file to an audio format that is supported by the device.

The files are unprotected mp3.

It used to work, then I installed a trial of Nero, and when I uninstalled it, I got a message in WMP that there was no mp3 encoder, which I downloaded and could rip cd's, but now get this error trying to sync.