In broad terms, you need to : 1) explicitly set the size and
position of the help window, instead of just using the default
settings; and 2) uncheck the option to remember the user-defined
window position after first use.
There is an HTML Help bug whereby, if you elect to use the
default window dimensions, any changes that a user makes to the
size and position of the window are remembered and recalled when it
is next opened — even if you've chosen to discard those