Google "chromakey" and you will probably find something pretty quick. It's a standard trick in video apps.
Corel has a plug-in for Photoshop called Knockout that specializes in this. http://www.corel.com...5061771#tabview=tab0
You don't even need a green screen to use it, so it should work great if you've got one.
A company called Serious Magic used to have an app called Ultra that did this for video. They were bought by Adobe who now sells it, but I don't know if it's stand-alone anymore. It may only be available as part of one of Adobe's suites.
You could also check on software from Ultimatte. They are the originals and the uber-pros at greenscreen, but their software is pricey. You might check out the Ultimatte plug-in for Photoshop, though. I think they have one.
Many video programs have this built-in - Premier for example, or Sony Vegas. You could also look into Zweistein, a video app that has chromakey and is not limited to video resolutions. It's also free. Google "Thugs At Bay Zweistein" and it'll come right up.