Ok, so you are using IIS right and your url is like http://localhost/mweb
First thing to check is if there is another Web.config file at the root, ie at the localhost level (which by default would be c:\inetpub\wwwroot). If so that is interefering so remove it if you can.
Second make sure your mweb virtual directory is configured for 2.0 .NET and using an app pool for 2.0 .NET
Hope it helps,
Joe