VS 2008 - Redirect To A Page If File Not Found In Project

I'm using VS 2008, .NET 3.5. I need to redirect to a specific page if a requested file for a certain directory is not found.

Here's the example:

http://mysite/myproject/

is the directory. I want it where if you try to navigate to a page that doesn't exist in that directory, it goes to a specific page, ie:

http://mysite/myproject/badpage.aspx

this page doesn't exist, I want it to automatically redirect to:

http://mysite/myproject/myNotFoundpage.aspx