DivX is not a lossless codec so converting back to its original source (probably MPEG2) will never be as equal as the original. Its like an MP3 compared to a WAV file, once convert you can never go back 100%.
Also, converting a DivX AVI to an un-compressed AVI will give about 15 Gb file per 1hr of movie. Hope your running big HD's NTFS formated.