Читаем дальше MSDN :
Код:
Windows 95, Windows NT 3.51: Support for the mouse wheel is
provided through a separately-running module, MSWheel, that
generates a MSH_MOUSEWHEEL message. The MSWheel module,
which consists of MSWheel.exe and MSWheel.dll, is installed with
the IntelliPoint software that is shipped with the IntelliMouse®
pointing device. In addition, MSH_MOUSEWHEEL is defined in the
header file (ZMouse.h) that an application must use to implement
support for the wheel via the MSWheel module.
MSH_MOUSEWHEEL
zDelta = (int) wParam; // wheel rotation
xPos = LOWORD(lParam); // horizontal position of pointer
yPos = HIWORD(lParam); // vertical position of pointer
Note, there is no fwKeys for MSH_MOUSEWHEEL. Otherwise, the
parameters are exactly the same as for WM_MOUSEWHEEL.