you can just download realplayer and then on every video u watch online it will give you an option "download video or record"
download real player on your pc. every time u connect with youtube you will got the option on the youtube player
I use firefox with the Video DownloadHelper extension https://addons.mozilla.org/en-US/firefox/addon/3006
Install FDM(Free Download Manager) on your PC. Just Copy The Link(URL) Of The YouTube Vedio And Paste It In FMD. Downloading Will Automatically Start.
u can download the youtube video by jst changing the name of the site to voobys... For example..... https://www.youtube.com/watch?v=F-YJhMnYUd4&feature=related NOW change it to https://www.voobys.com/watch?v=F-YJhMnYUd4&feature=related... The only thing u need to do now is to change its format... Enjoy