Can not map a login from Windows

Today, I was trying to create a Login into SQL Server 2008 instance. The group that I wanted to be mapped was "Administrators" Group on a machine name "DBServer".

I tried to create the login, but recieved the error message --

"Windows NT user or group Administrators not found".

It was a bit strange because we all know that it exists on all the windows machines. Then I referred to a very helpful Microsoft documentation regarding that, whose link I am providing here

http://support.microsoft.com/kb/324321/en-us

If you go through that documentation, it says that the local Administrators group should be mapped as "BUILTIN\Administrators", and not like "MachineName\Administrators".