Unable to start debugging on the web server. The project is not configured to be debugged

When trying to run/debug my ASP.Net web app from VS.NET (2003) I get this error:

Error while trying to run project: Unable to start debugging on the web server. The project is not configured to be debugged.

This was caused because I had renamed the virtual dir that the project resides in - and because of this IIS had decided to revert the dir from an application to a dir. 

To resolve simply make it an app again via the IIS Management tool:

1>Within the IIS management tool, select your virtual dir.
2>Right-click and select Properties
3>In the Properties dialog box, click on the Create button.
4>In the Execute permissions, select  scripts and executables.