uses
  shellapi;
 procedure InstallScreenSaver(const FileName: string);
 begin
  { Set this screensaver as default screensaver and open the properties dialog}
  ShellExecute(Application.Handle, 'open', PChar('rundll32.exe'),
  PChar('desk.cpl,InstallScreenSaver ' + FileName), nil, SW_SHOWNORMAL);
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
  InstallScreenSaver('c:\YourScreenSaverFile.scr');
 end;