There is a possibility that there is no web.config file for the project or there is no Execute permission for the project.
If you already have a web.config file, double check it's well-formedness.
If the web.config file is missing, add one to the directory that contains the ASP.NET application. You can copy one from the numerous samples applications available with the framework installation.
Make sure that the following section reads like this:
defaultLanguage="c#"
debug="true"
/>