If you want to download videos on YouTube go over to Savevideodownload.com. What I like about their service is that it is so easy. you don't have to download anything. All you have to do is go to their download page, paster the url of the YouTube video you want to download in the given space and click "Get Download URL". Save as a .flv file into your hard disk.
If you already don't have one, you need a media player that suppports .flv files.