I tried to capture a small clip from an mvd file to avi and use the command "demo_capture start 15 frag".
It records the AVi file but at the same time im getting this error warning "ERROR! Cannot write to AVI file!", spamming the screen.

Anyone know any solution? Im running ezquake through crossover games in snow leopard, could that be the problem?
it still writes the AVI file so i dont think so. suggestions?

install xvid codec and i suggest also installing lame encdoer for mp3
http://ezquake.sourceforge.net/docs/?video-capture

but running that thorugh crossover can be a major problem - wondering if you can register the codec so that crossover sees it.

i would rather go in capturing screenshots.

I have found that lowering your resolution helps fix this error. Try 640x480 for starters.