You need the following programs to download Youtube videos and build flash screensaver with them:
Flash2X Flash Hunter is able to download Youtube videos and save them as FLV files on your computer. FLV is a video format for flash. To learn more, please read How to download Youtube videos.
You can use Flash2X Screensaver Builder to create Youtube screensavers with FLV files you download from Youtube. The program generates executable installers for your Youtube screensavers so that you are able to easily install them on computers.