Well if g-spot says have all the codecs u need then it is probably so....because it does exactly what AviCodec does

It does however sound strange that avicodec just runs for a bit and then stops and does not give u any information at all.
I have to ask a few obvious questions....
1. have u tried re-instaling quicktime?
2. .... .. . tried un-instaling quicktime and just run media-player and vice versa.
After reading your post a bit more carefully i find that the most probable cause of your problem is some sort of conflict between the two players since u can see the thumbnails in your folders because then it is not a codec problem.
So un-install all players and codecs and try to download just one and go from there....that would be my approach.