Hi Sven,
Just a suggestion and I am not sure this is the one, but could it be 'The Last Starfighter'?
This film was released in 1984 and starred Lance Guest as Alex Rogan/Beta Alex and Robert Preston as Centauri.
Alex plays and wins an arcade game called The Last Starfighter and is then recruited by Centauri to protect the galaxy from invasion.
You can get more information by going to:
http://www.imdb.com/title/tt0087597/
You can see a trailer of the film there also, but you may have to endure a advertisement before the clip starts.
If this is the film you are thinking of and want to own it, it is available on DVD
I hope this has been of help.
Cy
(Poseidon)