Do you know that some videos, like the following one for example:
http://video.google.com/videoplay?do...44823518404383

can be downloaded by using a Download link at Google video

You may download .mp4 videos for Sony PSP and Apple iPod or download them in the Google's proprietory format to watch in the Google Video Player.

Downloading with WM Recorder (recommended)
WM Recorder stream capturing software can record Windows Media, RealMedia/RealVideo/RealAudio, QuickTime, Shoutcast, Flash video including .FLV video streaming from Google Video.

Downloading with Replay A/V (recommended)
Replay A/V stream recording/capturing software supports many types of media streams. Google video can also be recorded by it.

Grab from the cache folder of your browser
You can copy .FLV videos from the browser cache. See the following FAQ if you don't know how to find the cache folder of your browser:
How to record/download/capture/save Macromedia Flash .FLV video HTTP stream from embedded player

Download Google video with any download manager
Use URL finder to find a stream URL and download it with any download manager. Refer to the following FAQ for more
How to record/download/capture/save Macromedia Flash .FLV video HTTP stream from embedded player

Using web-sites to download flash videos
keepvid.com and javimoya.com/blog/youtube_en.php can be used to download flash videos from many web-sites (unfortunately not all of them), including YouTube and Google Video.

You may also use a free FireFox VideoDownloader plug-in by Javi Moya to download flash videos from YouTube, Google Video, iFilm,... The plug-in also uses web-site to download videos.
Visit VideoDownloader homepage.

I've tried both and there were unable to download video from the following URL
http://video.google.com/videoplay?do...80938991195179
So there could be problems when downloading Google Videos.

Using VDownloader - free flash downloader for YouTube and Google Video
VDownloader
VDownloader can download videos from youtube and google video in AVI format.

To download a clip you only have to enter the url of the page with the video you want to download, and the program will save it in your hard disk.
p.s. You might not like the lossy video conversion the program uses.

Video screen capture software
Don't think it is a good idea to use it.