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.

[adicoto]

Can you tell what codecs do you have installed, the video card you are using and if you have enabled under video properties overlay mode ?

[DjDeluxay]

For me the quality is bad in regular sized window.
Anyone know why?
When i play it in Windows Media player it plays fine on both regular and full screen.
I just use this for screen shots. and i wonder why the quality is bad
Any help? Are there codecs i need especially for Bsplayer?