As you may already know YouTube video files have FLV extention. If you download basic version of ClipExtractor you get FREE tool for downloading and converting those files to AVI. Just follow this link and download Clip Extractor Basic. After installing  this application just open Internet Explorer, find YouTube video you interested in and click the programs'  icon at the right side of your browser's toolbar. The pop-up window will appear with URL of the video automatically entered into the upper field. Choose AVI format, desired quality and output file destination. Then click "Start Download" button and the program will begin to download and convert the video. You can see how it's done in this short video.