If you performed an upgrade install of Vista overtop of XP, then Hal will not work unless you reinstall Hal. The Hal installation program does slightly different things during installation on Vista than on XP, so doing an install of Vista over XP bypasses this. But reinstalling Hal should fix it.
As for the strange appointment reminders, right click on Hal, go to the Hal's appointment book and delete the "What day" events from your calendar.