I notice you have been looking into way of keeping your favorite YouTube videos on your beloved hard disk in order to play them again in the future. Actually there is plethora of solutions to choose from, but up to now here is the most straight-forward method available.
A temporary file with all the content of the FLV (Flash Video) file is created in the temporary folder of Windows, usually located under [User Profile]\Local Settings\Temp.
In order to view this file, go to your temp folder while playing a video in YouTube or in other similar Flash video Web site, and click the ‘Date Modified’ column to sort the files list by date. Then you will find that the first filename begins with ‘FLA’ prefix and the file size is gradually growing while streaming (F5 to refresh).
This ‘fla???.tmp’ file is in fact a full .FLV file that can be played with any video software that support FLV files. However, this file is locked by the flash player component and you cannot copy or open it. Also, when you close the web browser, the file is automatically deleted.
In order to do just that, we need to unlock the file. The key can be downloaded here for free:
After installation, go back to your temp folder to find the file you want to keep (Images below). Right click onto the file and select unlocker. Position your cursor at down left menu and select copy. Click ‘Save’ to save the file to your destination folder. Go to that folder to rename and adding FLV extension to that file.
Alternative method: using a free Mozilla Firefox add-on (tested successfully by saving videos from Youtube and Vimeo per April 2011 on Firefox 4), such as: