If you have problems with rar archives, (sometimes the file is ok but winrar says it is corrupted or it is empty - a bug i think or something else) try this first. Download and install 7zip. It is freeware and better than winrar. If you still can not decompress the archive with this tool then the file has a problem ... :)
Also check this download tool here and this site. You can download videos from youtube and convert them to avi.