For the past couple years I've been using Phatsoft TMR, a little Windows utility that leverages the Windows Task Scheduler to pop up notifications at a specified time. It does what it does very neatly, and by dicking with tasks at the Task Scheduler level I can create notifications that fire daily but which can be postponed to re-remind me if I can't do whatever daily thing right that moment. The only thing I could ask for more, is if it also had some sort of interface to show a complete list of pending notifications, sort-able by time or subject.