I'm looking for a very simple program to be written that can be used under XP/Vista/Windows 7

It's a service reminder program that will pop up a message onto the screen when a certain period of time has elapsed since the computer was serviced and the program was installed.

Here is the scenario:

A client of mine brings me a computer to service and clean. After I do the service, I install the Service Reminder Program.
This Service Reminder Program runs as a service and is minimised to the tray or hidden.
After six months or a year, the Service Reminder Program will display a message on the screen to say that it is time to bring the computer back in for another service and cleaning.

The program will be reset after the next service to begin the countdown again.

Things I would also like:

To be able to change the duration (6 months, 12 months etc) and edit/choose the message when it is installed.

For the pop-up message to display a logo etc.

Access to the source code so I can change things later if I need to like the message, logo etc.

A 'Snooze' feature so the customer can click a button to be reminded again in another week etc.