To get movies on my my HTC Desire HD I first connect to my desktop PC in USB disk drive mode.
I don't like the Android default folder structures so I keep all my personal stuff in one place. I created a folder in the root of my SD card called Steven's Stuff and a sub folder called Movies.
I then dragged and drop some movies to the Movies folder.
I disconnect the HTC Desire HD from my desktop PC.
I start the Videos app and it scanned my whole SD card and finds all my Movies (also works for pictures).
So far the HTC Desire HD has played every movie file I have thrown at it and there has been no need to scale down to a smaller format for mobile phones. Very impressive.