Ok, so you have probably seen this like a thousand times, but I'm going to mention it again. If you're working on a web application, and for some reason, you change its physical location and make the virtual directory point to this new location, and then try to open it in Visual Studio .NET 2003, the application will not be opened and you'll get this screen
How to fix this? Delete the Visual Studio .NET web cache. You'll find it in [windows root drive]\Documents and Settings\[user]\VSWebCache\[computer name]\