Firstly you need firefox as this is an extension

www.firefox.com

Download it and install it, its free :)

Next goto addons.mozilla.org ( I think ) and search for videodownloader (should be on the top ten)

In firefox goto tools>extensions

Enable 'VideoDownloader'

Restart Firefox

Go to the youtube video

Find the icon that looks like a film and a diskette in the status bar

Download the FLV using it