How to create a video for playback with Windows/XBox/PS3/… etc?

download and install the free converter tool Mpeg Streamclip …

Step #1

export your Project from within iMovie (cmd-E) in any size you wish

Step #2

open resulting .mov with Mpeg Streamclip

Step #3

go straight to 'Save as' and select 'mp4' from drop-down menu

This process is very fast, because it doesn't convert the video, but re-wraps it.

The new .mp4 is compatible with any Windows flavor, XBox, PS3, Linux, …, etc etc ......

and, no, just changing the suffix .mov >> .mp4 doesn't do the trick. 😉