So you'd have at least four basic parts; a form you fill out to enter an appointment or thing to remind you of, a list of things pending, a background task that just watches the clock, and then the pop-up/scroller to do the reminding.
Is this Windows? Are you using VC? There are sample 'tray' apps that show you how to have a background program with an icon in the windows tray. Then you could click on that icon to get to the list and form. The background process would also be watching the clock (maybe using a waited-GetMessage() call).
There must be sample scrollers on the web as they're all the rage with MSN and other mail programs now. Google should help there.