{ 
  The function CheckCheckBox() checks or unchecks a Checkbox in another 
  window. 

  Parameter: 
  hApp : Handle to the parent window of the Checkbox. 
  ClassName: Class name of the Checkbox. 
  (For Delphi-Applications: TCheckBox. For C, VB,..: Checkbox) 
  bValue: Determines whether the check box is in the checked state. 
  CheckBoxNr: Number of the CheckBox (useful if there are several Checkboxes) 
} 

procedure CheckCheckBox(hApp: HWND; ClassName: string; bValue: Boolean; CheckBoxNr: Integer); 
var 
  i: Word; 
  hCheckBox: HWND; 
begin 
  if not IsWindow(hApp) then Exit; 
  for i := 0 to CheckBoxNr do 
    hCheckBox := FindWindowEx(hApp, hCheckBox, PChar(ClassName), nil); 
  if IsWindow(hCheckBox) then 
    SendMessage(hCheckBox, BM_SETCHECK, Integer(bValue), 0); 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  CheckCheckBox(Handle, 'TCheckBox', True, 1); 
  // Or / Oder 
  // CheckCheckBox(Handle, 'CheckBox', True, 1); 
end;