Well, this will get you the full user name, including the domain, but requires full trust:
System.Security.Principal.WindowsIdentity.GetCurrent().Name
So, you'd be able to substring off the domain by getting the info before the '\'. But again, since it would require your form to be full trust, I wonder if it would be worth it.
What about using user roles? Do you have an AD group that includes all the people who you would want to have *see* the section? You could have a user role that was the default, and hide the section if the user belonged to that role (i.e., a customer) and another role with the people who work at your company... the section will show for them.