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;