WMP9 screen is green while playing subtitles

I have an .avi file with the corresponding .sub. When .sub has a different name than the .avi, the movie plays normal with picture and sound. After I downloaded VobSub if I rename the .sub to match the .avi the whole screen is green (but I have subtitles :-)) as if a green layer is in front of the picture. If I rename the .sub (or the .avi), it plays normal, but no subtitles. Please help. Thx.