Recently, I converted a project from VS2003 to VS2008. The conversion deleted a file which wasn't needed in VS2008. When I tried to "rollback" the source from source control to run in VS2003, it wouldn't run.
Taken from the link above:
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>