Web Service - CSharp

I am learning about Web Services, so I am following the instructions in HOW
TO: Write a Simple Web Service by Using Visual C# .NET on the MSDN site:
http://support.microsoft.com/kb/308359/

I have entered the code but when I build it, I get an error about a file
that Visual Studio added called 'global.asax. This is the error:
c:\Inetpub\wwwroot\MathsService\MathsService\Global.asax: ASP.NET runtime
error: There is no build provider registered for the extension '.asax'. You
can register one in the <compilation><buildProviders> section in
machine.config or web.config. Make sure is has a
BuildProviderAppliesToAttribute attribute which includes the value 'Web' or
'All'.

The Help on buildProviders isn't clear and everything I have tried fails to
sort out the problem.

What do I do to get rid of the error message?
Why does Visual Studio include the file but not the instructions to run it?

Thanks for your help,
Helen