how to create a pop up "now processing" window

Does anyone know how to create a pop up window (eg. something with an animated gif or message that says "now processing") while the server side goes off and runs a long task. Once the task completes, I would like to be able to close the pop up window. The concept is similar to airline sites that go off and search for best available flights but leave the client with an animated gif.

 

Best way to do that in my opinion is to refresh the page every X seconds and look at the long task status each time.
You don't even need client scipting to achive that:
<META HTTP-EQUIV=Refresh CONTENT="5">

(But be sure to remove that tag whent the task is finished! []