I have found that whenever a file will not play, using the Limewire player, that if I right click on the file in it's folder pane and select 'open with windows media player', I will inevitably receive a message saying that 'windows media player cannot play this file because the file format does not match the file extension' or 'WMP cannot play this file because the file is corrupt or is not a valid file type'. Then I delete the downloaded file and find a different file (of the same song) and it will usually play just fine.
In a nutshell, the only thing I use WMP for is verifying file validity. ;?)