function ScreenSaver(SaverName: string): boolean; 
begin 
  if (not fileexists(SaverName)) 
  or (extractfileext(ansilowercase(SaverName)) <> '.scr') 
   then begin 
   result := false; 
   exit; 
  end; 
  result := 
   shellexecute(application.handle, 'open', 'rundll32.exe', 
   pchar('desk.cpl,InstallScreenSaver "' + SaverName + '"'), nil, sw_show) > 32; 
end; 
 
procedure TForm1.Button1Click(Sender: TObject); 
begin 
  if not ScreenSaver('c:\windows\system\natur.scr') 
   then showmessage('Fehler'); 
end;