Автор: http://www.swissdelphicenter.ch

uses
   Registry;

 {....}

 procedure TForm1.Button1Click(Sender: TObject);
 var
   Reg: TRegistry;
   KeyName: string;
   ValueStr: string;
 begin
   Reg := TRegistry.Create;
   try
     Reg.RootKey := HKEY_CLASSES_ROOT;
     KeyName  := 'htmlfile\shell\open\command';
     if Reg.OpenKey(KeyName, False) then
     begin
       ValueStr := Reg.ReadString('');
       Reg.CloseKey;
       Label1.Caption := ValueStr;
     end
     else
       ShowMessage('No Default Webbrowser !');
   finally
     Reg.Free;
   end;
 end;