JM,

   Well, the way that you describe it is really one of the few options that
you have.  Because you don't really don't have the capacity to have a push
mechanism, you have to address this differently (as you have pointed out).

   I don't know that you need a service here.  Granted, it sounds like you
are going to need to get information when the user isn't actively using your
program, but you shouldn't (and in Vista, can't) access the desktop through
the service.  So you will have to have a client program that is accessing
your service in order to provide the notifications.  Of course, this program
will run all throughout the user's session.  Because of this, it really
doesn't make sense to have a service.

   So, assuming you have that program running all throughout the user's
session, I would suggest that you download the schedule information when the
program starts, and then show notifications at the appropriate times (using
a timer, or some other mechanism).

   Now, if the user has the capacity to update this schedule outside of
this program (which it seems like it does), you will have to update the
internal schedule information (that is powering the timer) pretty
frequently.  You could make this a user-configurable option.  Of course, any
time your client program makes changes, that is a trigger to auto-update the
information.