There are a number of ways you can approach this.
You may find a simple multi-event "countdown" timer is best for your needs. This would be something that lets you "reset" multiple countdown timers as each one "expires". This would be helpful when you forget to take a pill for say 30 minutes, which would throw off your subsequent doses for that day. By using a countdown timer, you simply reset the "6 hour" timer when you take your pill, rather than schedule a pill for a specific time of day. The only "problem" with countdown timers is that typically you have to set them up manually at the beginning of each day.
(found out there's also a "RainlenderLite" available at
Very powerful and flexible. Overkill, but I'm pretty sure it would do what you need as I once looked at it for doing the same thing for managing my meds. However, I just ended-up using my PDA and a mullti-event "countdown" app.
I don't know if the freeware version has the functionality needed, but it looked interesting.
TimeLeft is a versatile desktop utility - it can be used as a countdown clock, reminder, clock, alarm clock, tray clock, stopwatch, timer, sticker, auction watch, work days/hours countdown clock and time synchronization utility. TimeLeft uses Winamp skins to show digits and text.
There are also extensions for various mail clients (e.g. Lightning for Thunderbird). If you use Firefox, you might also look at the Add-ons for Firefox.
Outlook, and Yahoo Mail used WITH Yahoo Messenger have timer/reminder functionality. I've never used them, just heard about them.
edit to add:
These got snipped off my post.
This is a kludgey way to do it, but it's probably not too difficult once you get the initial setup...set up.
Windows Task Scheduler, invoke a simple editor like Notepad and pass a parm of a specific filename (e.g. a file that says "Take pill xyz" at 21:00 hours). Can also specify sound files as an or/and event as well.
If you know how to write .bat files, you can simply have the scheduler open a window and "type" a given file that contains some specific text/sounds/etc.
There's also this,
edit to add:
There's also these...
List of freeware reminder/to-do programs. Some of them looked like they might be close to what you're looking for.
There's a freeware and a paid version
There's a ??? version and a "Full" version.