Is there a javascript out there somewhere that would do the following or is it just wishful thinking on my part?

Say for example that I have a 320X120 .swf that i want to include in one page on my site. But I don't want to write three different pages for different browsers - one for users with the Flash player, one for those without Flash and one for the dinosaurs who are using Lynx or Netscape 1.1. For those without Flash I want to insert a 320X120 animated or static gif in place of the swf. I don't want the viewer who doesn't have Flash to be prompted to download the plug-in. So the question is this -- is there a javascript that will say "if the user has Flash, insert swf; if the user doesn't have Flash, insert animated gif in same space; if user is still using an old browser, load a static gif" or something to that effect????