Error while trying to run project: Unable to start debugging on the web server

Open your Internet Information Services manager and from the [your machine]/Web Sites/ Default Web Site node select the virtual folder which stores your project. Right Click and select Properties to select the ASP.NET tab page. Make sure the correct version ASP.NET Version is selected.

I have VS.NET 2003 and 2005 beta 1 running side-by-side on my box and noticed ASP.NET Version 2.0 is targeted by default, even for VS.NET 2003 projects.