As Zoredeache mentions in the comment, there is no built in function there are some third party tools that can be found by Google search. I looked for documented "proof" from Microsoft that it does not exist, but on quick search did not find it.
Edit Given the constraints and goals you have mentioned. You should be able to meet your goal of sending reminders using outlook 2003, using delayed email delivery. You write an email to everyone who needs the reminder, then set the "Do not deliver before" time to what ever you want, click send, and your email arrives as the specified reminder time.