I have to say that I've never come across this issue before. I think that fidosdeader is probably right in that the file you are downloading is corrupt. I'm pretty sure there hasn't been a change in format of AVI's

but it could also be that you are missing a codec.
Just a few questions to try and narrow it down a bit

Are you having this issue with all AVI files you download, or just one in particular?
Does the sound play while it's showing the green screen?
Are you downloading the files with a download manager?
I'm assuming it's a direct download rather than a torrent, if you could copy and paste the link in here, or maybe PM me with the link to an AVI that you have downloaded and it didn't work, then I'll download it and see if it works for me.