.mp4 file won't open

[nukiduz]

hi there. i'm trying to open a mp4 file with VLC and Quicktime and i does not work. I've downloaded the last QT and VLC version and also de 3ivx codec but it keeps failing. QT just doesn't open the file and VLC can play it but video is too slow. It also gives me: ffmpeg: more than 5 seconds of late video -> dropping frame (computer too slow ?). I also tried mplayer but there's nothing new.
I have a 17" Intel iMac with 1gb. Do you know what can i do? thanks