All Windows screensavers are in a file-format ending in .SCR -- You can use static gifs and jpegs as wallpaper but you can't use a gif as a screensaver without running it through a utility which can convert it to an SCR file -- then you put the file in with all the other screensaver files and Windows will automatically list it in the screensaver selection panel. You can download a free trial of one such utility from: http://www.blumentals.net/scrfactory/