unit Unit1;
interface
uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs;
type
 TForm1 = class(TForm)
  procedure FormCreate(Sender: TObject);
 private
  { Private declarations }
  procedure AppIdle(Sender: TObject; var Done: Boolean);
 public
  { Public declarations }
 end;
var
 Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.AppIdle(Sender: TObject; var Done: Boolean);
begin
if GetAsyncKeyState(VK_SNAPSHOT) <> 0 then
  Form1.Caption := 'PrintScreen ia?aoa !';
 Done := True;
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
Application.OnIdle := AppIdle;
end;
end.

type
 TForm1 = class(TForm)
  procedure FormCreate(Sender: TObject);
  procedure FormDestroy(Sender: TObject);
 private
  { Private declarations }
  procedure WMHotKey(var Msg : TWMHotKey); message WM_HOTKEY;
 public
  { Public declarations }
 end;
var
 Form1: TForm1;
implementation
{$R *.DFM}
const id_SnapShot = 101;
procedure TForm1.WMHotKey (var Msg : TWMHotKey);
begin
 if Msg.HotKey = id_SnapShot then
  ShowMessage('GotIt');
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
 RegisterHotKey(Form1.Handle,
  id_SnapShot,
  0,
  VK_SNAPSHOT);
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
 UnRegisterHotKey (Form1.Handle, id_SnapShot);
end;