How to implement a timebased reminder?

[Tiemo Hollmann TB]

Hello,

I would like to create a timebased reminder (like a reminder from a calendar
app), which runs the whole day "in the background"/ as a task icon (windows)
or gadget and reminds me in settable intervals about anything.

How do you realize this kind of app with using a minimum of the processor
power? Using idle and idlerate or any kind of loop or perhaps something
completely different?

Which would be the less consuming approach?

Any hint welcome

Tiemo