[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
const
  SC_MOVE = 61456;
  MF_BYCOMMAND = 0;

type
  HMENU = THandle;

function DeleteMenu(hMenu: HMENU; uPosition: UINT; uFlags: UINT): BOOL;
  external 'DeleteMenu@user32.dll stdcall';
function GetSystemMenu(hWnd: HWND; bRevert: BOOL): HMENU;
  external 'GetSystemMenu@user32.dll stdcall';

procedure InitializeWizard;
begin
  DeleteMenu(GetSystemMenu(WizardForm.Handle, False), SC_MOVE, MF_BYCOMMAND);
end;