procedure TForm1.Button1Click(Sender: TObject);
var
hKey: Windows.HKEY;
cbData, Count: Cardinal;
begin
cbData := SizeOf(Count);
RegOpenKey(HKEY_LOCAL_MACHINE, 'system\currentcontrolset\services\mouclass\enum', hKey);
RegQueryValueEx(hKey, 'count', nil, nil, @Count, @cbData);
RegCloseKey(hKey);
ShowMessage(IntToStr(Count));
end;