C# - VS2010 Refuses To Compile A Namespace, Says It Doesn't Exist

I transferred my project into another computer from Win XP to Win 7. After installation, I realized that something in my App_code folder, called like mydata.web.utils namespace, has a class called like WebConstants (which is public class and public functions).

now in areas like global.asax, it tells me "WebConstants does not exist in this context"

Even though I have:

<%@ Import Namespace="mydata.web.utils" %>

And webconstants.cs has:

namespace mydata.web.utils
{
public class WebConstants
{
public static String APPLICATION_VERSION = "version";
........
}
}

No syntax errors or compiler errors in webconstants.cs

But everywhere else, it's saying "what is 'web'"??

If I rename the namespace from "web.utils" to just "web", still same problem. If I rename "web.utils" to "wez"--then the compiler errors go away.

But I cannot do this, because then I'd have to change it all over the project, which is a ton of work.

I also notice in the "build output" that there is no App_code being compiled in the project. Maybe because it's an ASP.net folder, I'm not certain. Is that normal?

How is it that the same source code, same visual studio, produces errors in win7 and no errors in the winXP computer?