You'll just need to set it to manual mode on the original computer and then just drag and drop the movie onto the iPad in iTunes. Unfortunately, doing this means you'll have to do that for everything - apps, music, etc. - if you want to keep that movie on your iPad, because once you switch off manual mode and then sync, iTunes will wipe the movie. It's more to your advantage in the long run if you just move the movie to your original computer if you plan on keeping the movie on your iPad for very long.
Or you can use something like airvideo, leave the computer on and stream it to the ipad. As long as you have a solid wifi connection the quality is exellent.
