I'd guess that the file is not really an mp3 file, but the file extension has been changed to mp3. You could check the ID3 tag of the file, if it has one, and you could also use a handy little program called EncSpot to check if it recognizes the file - get it here: http://www.mpex.net/en/software/download/encspot.html
. It should know if a file is an mp3.
If the file isn't an mp3, your Winamp probably has the plugin to play that type of files anyway, so you could copy them one by one to your XMPlay plugins folder to check which one it is, but note that any dsp-plugins won't work, only the in_*.dll types of plugins. Also XMPlay has to be restarted to make any plugins work.
Hmm, one more thing popped in my mind.. I'm not sure if mp3pro files could cause any problems (their extension is also mp3), because I don't remember any discussion about them, but I wouldn't think so
Hope this helps