_WIN64

The _WIN64 compiler directive should be defined when compiling you're application for the upcoming 64 bit Windows version. It affects the type of the dwData parameter of the HtmlHelp function. Here's what the Platform SDK has to say about it:

"Unsigned long type for pointer precision. Use when casting a pointer to a long type to perform pointer arithmetic."

Note:
"64 bit Windows 2000" is still under development and probably won't see the light of day until mid 2000. Beta versions are already available for Alpha and Itanium (scheduled for release in mid 2000).