computer log when external switch is flipped?

How would I go about interfacing my computer to the outside world? I'm
wondering if there's a way I can log the date/time when my USPS mailbox
gets opened. Which eventually translates to I have a wire with a hi/low
voltage for the mailbox is open/closed -- how do I connect that wire to
the computer? What sort of input port is suitable for such a thing?

And of course I'll need some software. (I hope I don't have to write
another device driver. It's been many years since I did that kind of
programming. Someone else must have already written something. Someone
else is probably already logging when their mailbox door is open/closed.)