You can download a demo copy of Adobe Premiere 6.5 from the adobe website, bear in mind that unlike doing this with a still photo in photoshop, achieving a good image without distortion requires well balanced lighting and an even colour temperature accross the background and so is often difficult to achieve. The easiest method to do this is to use flourescent tubes as these tend to produce a consistent light level across large areas, if you have to use standard lights then use something to reflect the light rather than shining it directly at the chromakey surface.
There is also a similar process to chromakey called 'Ultimatte' which is used a lot more in the professional enviroment which may achieve better results though again this depends on your starting footage.