yTimer2

This program lets you run an unlimited number of countdown timers, with a popup message and an optional audio alert when the countdown reaches zero.

You can also choose to run a program or open a URL in your default browser when the timer goes off.
Timers are specified in days/hours/minutes/seconds, and if you close the program with active timers it will automatically restart them with the correct remaining time when you reopen it.
yTimer runs in the system tray, so you can easily ignore it until the moment comes for it to unleash your reminder.