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: