The version of the Windows Common Control Library (comctl32.dll) installed on your system may not be able to properly display flat toolbar buttons. At the time of this writing, the version of the Common Control Library you have installed was distributed only with Microsoft's Active Access Pack, and the latest version of the common control library available from the Microsoft web site did not support all the Active Access Pack features.
We expect Microsoft will update the Active Access Pack and/or the Common Control library in the not-too-distant future. Read about the Windows Common Control Library for a pointer to up-to-date information.