in reply to
Re^3: Obtaining Windows username from an intranet ActiveState Perl script
in thread Obtaining Windows username from an intranet ActiveState Perl script
Because the users are on intranet and it's a Microsoft IT environment, their browser automatically identifies who they are (I believe using NTLM authentication).
Using ASP, the variable is HttpContext.Current.User
The original post that I linked to also describes this scenario (where the intranet user never has to supply their username/password to the site, but the site can still obtain it).