(*

 * Author  : http://www.michael-puff.de

 * Date    : 2006-03-29

 * License : PUBLIC DOMAIN

 *)

function GetUserNameEx(NameFormat: DWORD; lpNameBuffer: LPSTR; var nSize: DWORD): Boolean; stdcall; external
'secur32.dll' Name 'GetUserNameExA';

function GetLoggedOnUserNameEx(fFormat: DWORD; var Username: string): DWORD;

const
NameUnknown       = ; // Unknown name type.
NameFullyQualifiedDN = ; // Fully qualified distinguished name
NameSamCompatible = ; // Windows NT® 4.0 account name
NameDisplay       = ; // A "friendly" display name
NameUniqueId      = ; // GUID string that the IIDFromString function returns
NameCanonical     = ; // Complete canonical name
NameUserPrincipal = ; // User principal name
NameCanonicalEx   = ;

  NameServicePrincipal = ; // Generalized service principal name
DNSDomainName     = ; // DNS domain name, plus the user name

var
Size              : DWORD;

  Buffer            : array[..] of Char;
begin
Result := ;

  Size := sizeof(Buffer);
if GetUserNameEx(fFormat, Buffer, Size) then
SetString(Username, Buffer, Size)
else
Result := GetLastError;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
Username          : string;

  err               : DWORD;
begin
err := GetLoggedOnUserNameEx(, Username);
if err = 0 then
ShowMessage(Username)
else
ShowMessage(SysErrorMessage(err));
end;

http://www.michael-puff.de/Programmierung/Delphi/Code-Snippets/GetUserNameEx.shtml