Only Quicktime runs .mp4 files. Why?

[kimiraikkonen]

I have downloaded some amount of mp4 files and i have 3ivx installed.

But the problem is: These .mp4(extension) files does not run in WMP, Bsplayer runs only sound but no video.

But Quicktime Player runs with video and audio both. What are the suggestions? Is it about Nero Digital Codec(not present on my computer right now)?

Thank you.