Try the following with Automator and see if it meets your needs. These directions assume that you are running the version of Automator that shipped with Snow Leopard. If not it should still work but you will see a slightly different screen to start when you launch Automator.
1. Launch Automator and when asked what you want to create choose "Application" You will now be at the main Automator window.
2. The window has several panes including one with a search box similar to Spotlight. In the search box type "Get Specified Movies". Drag the action that it finds into the area on the right.
If that action does not open to show an area for entering the name of your movie, double click the title bar for that action to expand the box. Use the Add button at the bottom to select the movie you want to play or drag the movie into this box.
3. Now enter "Play Movies" in the search box and put that action in the pane on the right. Expand the box if necessary to choose the playback size, display, and number of times the video will loop
4. Go to the search box, type "Quit Application", and drag that action below the others. In the drop down menu select Quicktime Player".
5. Click the run button at the top of the Automator window to test your work. Ignore the warning that appears. If all behaves as expected choose Save As in Automator to save your work.
I recommend saving it twice once as a workflow and once as an application. If you want to use a different video simply open the workflow version, choose a new video and save the results as an application. Add the Application version to your login items and it should play the next time you log in.
Notes: I used QuickTime Player but it could be modified to work with other players. I also did not make any volume adjustments to the movie though that should be possible if needed. If you get stuck let us know and I will see if I can send you the workflow