Question : ASP.NET Modal popup window

I am developing in Visual Studio 2008 and am looking for a way to open a new modal window when a button is clicked. What is the best way to achieve this. Is AJAX based modal window an option? If yes, can someone guide me through the process of how to download/instal the compatible AJAX framework and so on...

Answer : ASP.NET Modal popup window

I have resolved this issue by myself. Thanks for your help MrAgile though. I deleted the Ajax Extensions tab that was already existing and might have old references. Then I re-added the tab and repeated the whole process. This time I do not get the above error.