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var shooting: Boolean = False;

var tmpTime: Cardinal;
begin
  if shooting then
    Exit;
  shooting := True;

  //Schuss anzeigen etc.

  tmpTime := GetTickCount;
  while tmpTime + 1000 > GetTickCount do
    Application.ProcessMessages;
  shooting := False;
end;					

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procedure OnTimer(Sender: TObject);
  ...  
  if (GetKeyState(Ord('A')) and (1 shl 31))<>0 then  
  begin
    Caption:='A Gedrückt';
  end else
  if (GetKeyState(VK_SPACE) and (1 shl 31))<>0 then
  begin
    Caption:='SPACE Gedrückt';
  end else
  begin
    caption:='';
  end;
  ...
end;					

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public
 procedure WMHotKey(var Msg : TWMHotKey); message WM_HOTKEY;

implementation

// im oncreate -> RegisterHotKey(Form1.Handle, vk_space, 0, ord(vk_space));
// in ondestroy -> UnRegisterHotKey(Form1.Handle, ord(vk_space));

procedure TForm1.WMHotKey(var Msg : TWMHotKey);
begin
if msg.hotkey = ord(vk_space) then begin
//.....
   end;
end;