Reminders quit working in Outlook ’98

I have one user out of 60 whose Outlook ’98 program will not display his calendar reminders. When the reminder is due to pop up you will notice the hour glass briefly and hear the computer as if it is doing soemthing but the reminder never appears.If I reinstall the program they will work until the next time he shuts off and boots up his computer. Ideas??
Thank you. 🙂