[NumLock]

Код:

uses Unit2, Unit3, Unit4, Unit5;
...
function Check(const User, Pass: String): TForm;
begin
  if (User='админ') and (Pass='sysop') then Result:=Form2
  else if (User='секретарь') and (Pass='masha') then Result:=Form3
  else if (User='бухгалтер') and (Pass='nomoney') then Result:=Form4
  else if (User='итд') and (Pass='anykey') then Result:=Form5
  else Result:=nil;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  AForm: TForm;
begin
  AForm:=Check(ComboBox1.Text, Edit1.Text);
  if AForm=nil then ShowMessage('Oops')
  else AForm.ShowModal;
end;