While this document says
HDF_SPLITBUTTON is "Microsoft Windows Vista and
Version 6.00", other references disagree:
http://msdn.microsoft.com/en-us/magazine/cc163384.aspx
http://msdn.microsoft.com/en-us/library/bb775339(VS.85).aspx
both imply that SplitButton is VIsta-only, not 6.00 (XP).
The same seems true for
HDF_CHECKBOX, HDF_CHECKED, HDF_FIXEDWIDTH and HDF_FOCUSED, which do not appear in the XP SDK commctrl.h.