1. How "large" is the green stuff? Is it covering like 5% or 25% of the screen?
2. What kind of video format are you trying to watch? i.e. is the file's name like "prisonbreaks1e4.wmv" or .avi and so on...
Because if it is in .mp4 (mp-format), the green stuff might very well be a failed update of that specific area in the movie-file. This is partly how the .mp#-codec works.
We'll have to get back on that if the format is a mp#-format.
3. Does this happen to all movie-files?
If it does, then it's probably not the codecpack.
Finally, provided you have more than 1 media player (if not; ignore question #4).
4. What Media Player does/does not show this problem?
This can also happen in YouTube-videos by the way.