function GetUserFromWindows: string;
var
 UserName : string;
 UserNameLen : Dword;
begin
 UserNameLen := 255;
 SetLength(userName, UserNameLen);
 if GetUserName(PChar(UserName), UserNameLen) then
  Result := Copy(UserName,1,UserNameLen - 1)
 else
  Result := '';
end;

function GetCurrentUserName: string;
 const
  cnMaxUserNameLen = 254;
 var
  sUserName: string;
  dwUserNameLen: DWORD;
 begin
  dwUserNameLen := cnMaxUserNameLen - 1;
  SetLength(sUserName, cnMaxUserNameLen);
  GetUserName(PChar(sUserName), dwUserNameLen);
  SetLength(sUserName, dwUserNameLen);
  Result := sUserName;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 begin
  ShowMessage(GetCurrentUserName);
 end;

function GetCurrentUserName(var CurrentUserName: string): Boolean;
 var
  BufferSize: DWORD;
  pUser: PChar;
 begin
  BufferSize := 0;
  GetUserName(nil, BufferSize);
  pUser := StrAlloc(BufferSize);
  try
  Result := GetUserName(pUser, BufferSize);
  CurrentUserName := StrPas(pUser);
  finally
  StrDispose(pUser);
  end;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 var
  CurrentUserName: string;
 begin
  GetCurrentUserName(CurrentUserName);
  Label1.Caption := CurrentUserName;
 end;
 {*********************************************}
 {
 Windows NT/2000/XP:
 The GetUserNameEx function retrieves the name of the user or other
 security principal associated with the calling thread.
 You can specify the format of the returned name.
 If the thread is impersonating a client, GetUserNameEx
 returns the name of the client.
}
 const
  NameUnknown = 0; // Unknown name type.
 NameFullyQualifiedDN = 1; // Fully qualified distinguished name
 NameSamCompatible = 2; // Windows NT® 4.0 account name
 NameDisplay = 3; // A "friendly" display name
 NameUniqueId = 6; // GUID string that the IIDFromString function returns
 NameCanonical = 7; // Complete canonical name
 NameUserPrincipal = 8; // User principal name
 NameCanonicalEx = 9;
  NameServicePrincipal = 10; // Generalized service principal name
 DNSDomainName = 11; // DNS domain name, plus the user name

procedure GetUserNameEx(NameFormat: DWORD;
  lpNameBuffer: LPSTR; nSize: PULONG); stdcall;
  external 'secur32.dll' Name 'GetUserNameExA';

 function LoggedOnUserNameEx(fFormat: DWORD): string;
 var
  UserName: array[0..250] of char;
  Size: DWORD;
 begin
  Size := 250;
  GetUserNameEx(fFormat, @UserName, @Size);
  Result := UserName;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 begin
  Edit1.Text := LoggedOnUserNameEx(NameSamCompatible);
 end;