There is software that can crop the avi so you'll get it down to a manageable size for Registax.
Virtualdub can do it. "Castrator" can do it too, and roughly aligns the planet as a bonus.
With the big avi, try using Autostakkert.
Or you could record several films, do several images and use Winjupos "Derotate images" feature to cumbine them.
Other solution, get a webcam. Even the old SPC900 does the trick. I also read there are PointGret Firefly models very cheap on ebay (there is a thread arround here about that).
Anyway, once you figure out how to use more frames, only seeing is the limit, if the scope is collimated.
Here are my Jupiter images from 2011 with a 6": http://i40.photobuck...upiter_2011.jpg