Problem converting song

Hi guys, I'm experiencing a very strange problem. I dunno if it is associated only with audio files, so feel free to move it if you think this is the case.
But let's get straight to the point. When I try to convert a file using Foobar it gives me an error saying it can't load the file, even if it is correctly played. The same with winlame, the conversion has no success. Moreover if I try to add it to mediamonkey's library, it isn't load. So it is not a specific-program problem, but a more general one.
Any ideas on how to solve this ?? I anticipate that the files don't have the "read only" attribute

Ps : sorry if I made some english mistakes