Make sure your video and audio have been properly transcoded before trying to burn a DVD. If exporting from PrPro, use the MPEG2-DVD format. If importing an AVI file into Encore directly, ensure that it is transcoded befor you save it as an ISO file.
The file you should have in Encore, after transcoding in Encore or exporting from PrPro should have an M2V extension for video and a separate audio extension (WAV, AC3, etc.). Once you have a proper file in Encore, "build" it as an ISO image, then burn a DVD with ImgBurn as previously stated.