This mini tutorial explains how:

1- Go to a movie on google video.

For example: http://video.google.com/videoplay?docid=-6256867741680867480

2- Wait until the movie is loaded (the progress bar turns from white to grey)

3- When to movie has beed downloaded (the progress bar is grey) go to:
C:\Documents and Settings\user\Local Settings\Temporary Internet Files

4- the flv file corresponding to the good flv video starts with:

videodownload?version=0&secureurl=

You can notice that the properties of the file give the url of ton file on google's server.

here: http://vp03.video.l.google.com/videodownload?version=0&secureurl=swAAAPAxh7pju8iX6mc3_NuA8rGtehxMgoPrtDNPyd7f7Xcfz89obd6CoGgIbWefgBpF_nhvds6VxxZUhh6ktq4ZaaYNe_DZmDIfjXlMA72xkQuTkc0Ip5UqNP5yC5tJtApqTrzMd8r9YLPDlJYzHx7UKvJaXSlXFuV5jbjBONFy6WgAB_buDKjiPynrOpnjm1B4OE8Nb_JDd-XkD9xs4VssspL8MEB4bX2ExXpD18gl2XKNyATHusfcowT-WmcMR3M4eA&sigh=GhsyImUNzwxDG9xodSFGCtgpHrQ&begin=0&len=84184&docid=-6256867741680867480&rdc=1

5- If you want read flv file, you can read them with FLVplayer, or Riva's player. You could also convert flv to avi files using SUPER, but it takes usually more space on disk.