[Selene]

Código Delphi [-]
Function NombreUsuarioWindows: String; 
var
   pcComputerUser : PChar;
   dwCSize    : DWORD;
begin
     Result := '' ;
     dwCSize := MAX_COMPUTERNAME_LENGTH + 1;
     GetMem( pcComputerUser, dwCSize );
     try
            if Windows.GetUserName( pcComputerUser, dwCSize ) then
               Result := pcComputerUser;
     finally
            FreeMem( pcComputerUser );
     end;
end;

espero que te sirva !!!