VS.NET 2003 Error The project you are trying to open is a Web project. You need to open it by specifying its URL path.

I'm working on a build server at the moment, and in the process of moving folders around, I keep getting this error in Visual Studio.NET 2003 when the location of a web project within a solution does not match the URL on the local machine. 

To fix this error all you need to do is add a webinfo file.  If your using VSS for source control, it's probably a good idea to add the webinfo file to SourceSafe as well as this is not done for you automatically.

If you have a project file c:\Inetpub\wwwroot\Folder\fred.csproj then you need a corresponding webinfo file fred.csproj.webinfo.  The contents of the webinfo file looks like this:

<VisualStudioUNCWeb>

    <Web URLPath = "http://localhost/Folder/fred.csproj" />

</VisualStudioUNCWeb>

When you go to add this web project to your solution file you actually don't specify “Add Existing Project From Web”, but simply “Add Existing Project”, then browse to c:\Inetpub\wwwroot\Folder\fred.csproj

For some background information on this problem you might want to have a look at Web Projects and Source Control Integration in Visual Studio .NET or Team Development with Visual Studio .NET and Visual SourceSafe

Here is an extract:

WebInfo files (*.csproj.webinfo or *.vbproj.webinfo). This file keeps track of a project's virtual root location. This is not added to source control to allow individual developers to specify different virtual roots for their own working copy of the project. While this capability exists, you and all team members are recommended to use a consistent (local) virtual root location when you develop Web applications. The recommended structure for Web and non-Web applications is discussed in Use a Consistent Folder Structure for Solutions and Projects.