After Vista re-install, can't add .flv, .avi videos to MediaPlayer library?

After a clean re-install of Vista Home Premium (on a Toshiba Satellite L355D), I am no longer able to add .flv,.avi videos to the "All Videos" section of the MediaPlayer library. I can easily play these videos individually by opening the containing folder and then clicking on the videos one at a time (kimcode and ffdshow codec packs are installed on the machine), but when I attempt to drag the entire contents of the containing folder (about 1,000 videos) into the "All Videos" section of the MediaPlayer library, only about 60 (presumably, only the ones whose extensions are "recognized") of the 1,000 get listed there, the rest are just ignored. I cannot get around this problem by using "rename" to change the file extensions to .wmv, as I could before the re-install; this was easy before, it doesn't seem to be possible now!