Problems of vision

when i see an avi, or mpg film I have a big problem. If I see it in a small window there isn´t any problem I see it ina crystal clear quality.... but if i try to see the video in a bigger window, the quality dissapear and i see a lot of big green points.
Is there someone who can explain me why I have this problem?

Thank you, and sorry if I don´t write well in English... I´m Spanish.