Welcome to the MacNN Forums.

This just happened yesterday, everything was going fine until I tried to import an AVI from my camcorder into iMovie (the same camcorder i've been using for weeks with no problems) and all of a sudden I get

"You need to download additional software to play this file" etc. And the screen is white.

Obvious question: have you installed anything lately that could have messed it up?

The only thing I can think of is VLC Media Player, but that wouldn't mess up Quicktime, as it hasn't in the past.

EDIT: One more thing I just thought of, there was a setting to make Quicktime my default player for internet videos or something like that, and I said yes, I don't know why that would make it not play AVI's anymore though.

Open the avi in VLC and get info to see what the video fourcc is.

Then list the components you have installed in Library/Quicktime and Home/Library/Quicktime.

Ok the problem is fixed, thanks Jacke. Apparently, somewhere along the software update that I did, a xvid codec and a mpga audio codec got deleted? It was working fine until I did a software update.

I installed xVid and Perian, and everything's working now. Much appreciated.