If you have an Apple TV, you can use a computer on your home network to stream movie files to your television.
First, you will want to add the movie files to your iTunes library (on the computer you are going to use to share your media). In the iTunes preferences, go to Advanced Preferences and make sure Copy Files To iTunes Media Folder is unchecked (assuming all of your movies will be on an external hard drive). Then, click on File - Add Folder To Library and browse to the location of your movies folder. Depending on how many movies you have, this could take a significant amount of time to process.
Once this process is complete, you will want to enable sharing of your media library. Back in the iTunes preferences, go to Sharing and make sure Share my library is checked. You can choose to share only certain things (such as Movies) or everything, and you can also choose to password-protect your shared media.
This method is an older method known as "Music sharing" -- nowadays, using "Home Sharing" is a much simpler method. Here is a guide on how to configure (it basically just involves turning it on, no complex setup needed).
Once this is done, your computer is now set up to share out your media library to your Apple TV. Keep in mind that your computer and Apple TV will need to be on the same network, and the computer will need to be powered on whenever you want to stream your media.