Windows Media Player error : file in use

Hope anyone can help me with the following :

I have several windows media files (.wmv) on a Apache webserver. When workstations (windows xp sp1, media player 8) try to open the file, Media Player returns with an error: unable to open file, file in use.
Filerights are ok. I moved the files to an IIS server, but with the same result.

I hope anybody can help me.

Thanks !