I highly recommend playing h264 HD videos with VLC get it here: 
http://www.videolan.org/

I do not recommend playing with QuickTime/iIunes as QuickTime/iIunes is a dog and sucks at h264 decoding.  Only play with QuickTime/iIunes if you have a really powerful new PC.  VLC uses like 1/4 of the cpu power to play the same video.

Plays fine on PS3, Xbox 360, Popcorn Hour, old pentium 4's (with VLC), and Apple TV most likely.

-HR

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