I use FRAPS, the freeware version. I don't make movies so it is perfect for screen shots. You can choose image format, .bmp, .jpg, .png, or .tga, as well as what key
you would like to use to capture the shot. You can also designate where the shots will be saved and it has a tic box to repeat until pressed again. You type in the number
of seconds you want to pass before it repeats. It will also make 30 second movies.
http://www.fraps.com/