Have you
- restarted your PC
- searched for the plugin .dll files - still there or not?
- reinstalled the plugins?

You haven't updated from Nero5 to Nero6 and now it's broken, have you? (Plugins are stored in another directory now IIRC.)

In case you need to burn / playback some mpc files urgently, you could use foobar2000 (http://foobar2000.hydrogenaudio.org) until the problem is fixed (for playback and for creating .wav files from mpc for burning using diskwriter).

I'm not really having an idea ... .
As you're talking about Nero and Winamp I guess there's a playback/decoding problem rather than a encoding problem. Have you tried if the .mpc files you want to play back/burn are alright? Are they decodable using mppdec.exe or foobar2000?

What exactly are the problems like?
- What happens if you drag'n'drop a .mpc file to Winamp's playlist?
- If you try to open files with e.g. Nero Wave Editor - are .mpc files recognized as valid audio format at all, and -if yes - what exactly happens if you try to open them?

Could there be a problem with Windows' file associations?

(Some of these ideas could make no sense but better than nothing ;) ). You might want to ask for help at http://www.hydrogenaudio.org forum; probably some people there have better ideas than me ...