I've had the same problem, cept its with flv's, my player has a test flv video and it plays but when i download from youtube it won't play, so goes for avi etc.
i used this to convert to avi thats
Pazera_Free_FLV_to_AVI_Converter.zip
still funny though, i still need to convert avi files into avi files again with it so it will play