Movie Problem.

Hello all i am new to the forum and i was jsut needing some help with movies. ATM i just spent a couple days downloading a movie(i am in iraq so therefore my net blows) anyways. While downloading this movie i double clicked it so i could preview it and see what quality it was. Well in the process of doin that it was good quality and well now that the whole file is downloaded. I can only watch the part that i previewed. I dont know if anyone else has this problem but can you help me pls. And yes i am working on Mac OSX that is why i posted here.