You won't need anything. There is so many ways, and what should you choose is depends on what do you want to get. If you just want to have a birthday reminder, follow Smac89 answer, else if you are using windows re a book on WinAPI to i.e. create a program that runs as a service and if it'll find a birthday today, it will bring window with reminder and suggesting to send a birthday card from your mail account. There is no definite "right" way to do that. Some are easy, but "dirty", some are harder but it's reccomended by OS developers to do that, some looks like an overkill for a simple program.
You can create a program which will check birthdays and place it in windows autoload folder (so it checks birthdays on each launch) and shedule it in windows planner for 11.00 each day (so if you didn't turn off your PC it will still check birthdays on that day)