Error: 'Unable to access XXX' possibly media player has this file open

[Unregistered]

I keep getting this error message while trying to convert the bitrate of specific MP3's. I know they're not currently opened by any media player... I've rebooted to make sure and still get the same message after a fresh reboot. Something stinks. Any ideas on how to fix this or why I'm getting this message? Thanks

[Spoon]

Convert them to a different folder, or make sure they are not read only.