Download a player for called VLC ( which is a free 'Flash Video' player)
-Play the YT vid u would like -copy the address by highlighting (click-drag the mouse) right-click or ctrl-left click the mouse
-select 'copy' from the resulting sub-menu open a new tab after you have bookmarked the 'keepvid' link below and again right-click or ctrl-left click the mouse select 'paste' in the keepvid window and follow the site's dialogue and menu.
I know that all sounds complex- but you will get used to right-clicking or ctrl-left clicking the mouse and making extensive use of it throughout your computer activity. And save yoursel a lot of typing -'copy & paste' everything you can including pictures and graphics !
http://keepvid.com/