AviSynth is a frameserver program…an intermediary between the video and an encoding program, or media player. It can be used for any number of purposes, including changing resolution, deinterlacing, cropping, adjusting color, contrast or brightness, etc. It is a very powerful script based program for working with video. But it is not, in and of itself, a conversion program.
I suggest you look into a program called AutoGK for converting dvds into avi format using the xvid codec. There are many other choices for this type of conversion, including SUPER, FairUse Wizard, Xvid4PSP, Handbrake, and more. You can find a list of such conversion programs over at www.videohelp.com
By the way, DVDDecrypter is no longer updated and is ineffective for decrypting many newer movies. If you run into one that it cannot handle, you should get DVDFab HD Decrypter. It is the free section of DVDFab, and will continue to work even after the free trial of the main part of DVDFab expires. http://www.dvdfab.com/free.htm