two possible solutions:
1) check isapi associations for aspx files in IIS.
2) try to unregister .net isapi extentions and reregister them again. This is done with regiis command line utility in the .net framework folder.
>I am working on a Visual Studio 2005 project that uses the ASP.Net
>Development Server. When I run the VS project (Debug / Start
>Debugging or Debug / Start without debugging), it starts the ASP.Net
>Development Server and also tries to open the aspx file in EditPlus.
>That causes an error, which is understandable.