You can use this "Task Scheduler" facility in Windows 7 to create a one off reminder or daily, weekly, monthly, yearly or whatever you want:- http://freewindowsvistatutorials.com/setupRemindersInWindows7OneTimeNotificationsRecurringAlerts.php
If you have the outlook e-mail client then you can set appointments to alarm from there. You have to have your PC on at the time though, to see the alarm.
Failing that there must be a multitude of progs out there to download.
Thanks Dodger666, that might well do the business. Do you know what happens if I haven't got the computer on at the scheduled time? Will it activate next time I put the computer on, or will I have missed it?
Any task you schedule occurs(triggers) on a date/time you set when creating the task. You can set an "expire" date for the trigger to expire. In your case for a one off reminder you could set the trigger on the date you want it to appear and expire a week later or whatever.