This is a quick and easy trick, you just have to:
1 - Go to the YouTube page.
2 - Search for the video you want to download.
3 - Copy the URL from the address bar.
4 - Go to DownloadYouTube.
5 - Paste the YouTube address where indicated and press the button "download"et voilà.
6 - (Optional) The downloaded file will be in *.flv (Flash) format, to change it to *.avi (Movie), and be able to see it in your normal player, I leave you here a link to a program so that you can change its format.
This is one of the ways to download videos from YouTube of the multitude of ways that exist, but I think it is the simplest, I hope you find it useful.