ctrl-alt-del

procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  SystemParametersInfo(SPI_SCREENSAVERRUNNING, INTEGER(CheckBox1.Checked), @i, 0);
end;

Husk at tilføje WinProcs under uses

uses
  WinProcs;

{$R *.RES}

var
  Dummy : integer;

begin
  Dummy := 0;
  {Disable ALT-TAB}
  SystemParametersInfo( SPI_SETFASTTASKSWITCH, 1, @Dummy, 0);
  {Disable CTRL-ALT-DEL}
  SystemParametersInfo( SPI_SCREENSAVERRUNNING, 1, @Dummy, 0);
end.

Hvis det er en Win-NT, Win-2000 eller Win-XP
så se endvidere http://www.eksperten.dk/spm/107849