When you open a Visual Studio .NET 2003 web project, you need to have your IIS virtual directory match what is in your SLN file as well as your .webinfo file. That IIS virtual directory needs to also map to the actual directory your project file is in.
Whenever you attempt to open a web project (csproj / vbproj) in VS2003, it will attempt to read from the VSWebCache reference first. This can cause your development environment to break, if you happen to have moved files around since the last time you opened your project.
Browse to the VSCache location for your user and clear out the appropriate folder to reset this. The location is set in VS under Tools –> Options –> Projects –> Web Settings. See screenshot below.