This happened to me, same error. I'm working pretty consistently between a few sites simultaneously, all running as virtual roots off my c:\inetpub\wwwroot folder. Somwhere along the line, some configuration or site setup process installed a web.config in my wwwroot folder with this xmlns attribute added to the root node.
My application was running off a virtual directory at: http://localhost/clientroot
I didn't need this web.config file in the wwwroot b/c i'm not running any apps off that folder directly. They're all virtual directories off the wwwroot. I deleted the web.config in my wwwroot folder, and my problem dissipated. I was then able to run and debug my "clientroot" project properly in VS2003.
The thing that clued me off to this was that I was attempting to load my clientroot project in VS2003 and debug it. The url in my browser was http://localhost/clientroot, but the error message that came up noted a problem with web.config located in c:\inetpub\wwwroot. This told me there was some sort of configuration issue with IIS running the web.config in the root folder, not my project folder.