Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

procedure TForm1.Button1Click(Sender: TObject);
 var
   ncm: TNonClientMetrics;
 begin
   ncm.cbSize := SizeOf(TNonClientMetrics);
   {get old non client metrics}
   SystemParametersInfo(SPI_GETNONCLIENTMETRICS, SizeOf(TNonClientMetrics), @ncm, 0);
   { 
  lfCaptionFont - regular captions 
  lfSmCaptionFont - small captions 
  lfMenuFont - menus 
  lfStatusFont - status bars 
  lfMessageFont - message boxes 
  }
   ncm.lfMessageFont.lfFaceName := 'Tahoma';
   {set new non client metrics}
   SystemParametersInfo(SPI_SETNONCLIENTMETRICS, SizeOf(TNonClientMetrics), @ncm, 0);
 end;