The original iPad is able to run the latest version of iMovie for iOS, even though, Apple has decided to not allow original iPad users to install iMovie directly through the App Store.
"Designed for the Multi-Touch screen, iMovie puts everything you need to tell your story at your fingertips. Browse and play projects in the stunning Marquee view. Make a movie in minutes by adding video, photos, music, and sound effects. Then give it the Hollywood treatment by adding a theme, titles, and transitions. And you can share your movie with the world -- right from iMovie."
Original iPad users who want to run iMovie on their original iPad can get iMovie without a jailbreak. The first thing you need to do is install the latest version of the iPhone Configuration Utility software (Mac and Windows), which was updated to version 3.3 yesterday.
One done, now open the iPhone Configuration Utility, click applications on the left side and click open iMovie. Then connect your first generation iPad to your computer, and then click the install button in the configuration utility software. Now you should be good to go.