The vast majority of torrented movie files are not legal, and not a subject for discussion around here, but on the very large assumption that your files are not copyright protected, we may be able to help.
Nero Vision, within the Nero 8 Burning Suite, should be able to convert avi to dvd for you. It is far from my favorite tool for this however.
Try DVDFlick, AVStoDVD or FAVC. All are free to download and use.
http://favcfavc.googlepages.com/home (you'll need AviSynth and Net 2.0 for this, but both are free and there are direct links to both at the FAVC homepage)
One popular commercial product for this conversion is ConvertXtoDVD.