Read http://www.afterdawn.com/guides/ . The quickest is CCE , but it is the most expensive and the most difficult to use. A quick way is to make (S)VCD movies (few hours), and its the suggested waty to do with an AVI with average quality. More, a DVD , if you author an image with DVD Lab, can contain many movies (800 MB = 80' VCD = 55' SVCD). Try to make VCDs and compare them with what you use for DVD. You'll see I'm in the truth.
Try convert the avi to dvd compliant format with e.g. VSO DivxToDVD, then burn with e.g. Nero: http://www.afterdawn.com/software/video_software/video_encoders/vso_divxtodvd_free.cfm
