When you say microsoft windows, I am assuming you mean the default calendar that comes with Windows XP or older. I agree, if you dont have outlook, outlook express or Windows Vista, Hals calendar is a step up. And it is good that Hal has a basic defacto calendar. Even if someone added exchange client or DDE support to Hal, there is a possibility that none of the supported calendar programs are on the users machine.
But... Hals calendar is still about 3 generations behind outlook or even the default calendar program that comes with Vista. (Looks like it is strait out of the 90s with only the most basic functionality). Outlook and Vista's Calendar support tasks versus appointments, color coded priorities, various views (weekly, daily, monthly), integration across other MS Office products and cutomizable reminder options. Sorry, but three days just doesn't work for every appointment type in the world. Some things you dont need to know about till the day of. Other things, you need to know about weeks in advance.
And while yes, Hal could be upgraded... why? Let Hal focus on what hal is good at. Let someone else build the calendar/appointment management program. Just make Hal capable of pushing and pulling data from it. Im telling you, this functionality is money in the bank. There is NOTHING on the market that does this right now. Dragon NaturallySpeaking comes the closest, but it doesn't support natural language for desktop command and control. You have to use strict commands.
Anyway, thats my 2 cents. I may look into this.