Use Computer To Control A Simple Alarm System?

I have a project for a course in school where I have to develop an interface for a specific task. The task that I would like to choose is a simple home alarm system.

I would like to essentially control a simple device by my computer. I am not sure what would be the best method of connecting the device to my computer, be it USB, Serial, or Parallel.

I would like to create a sort of alarm box with sensors, I can arm  / disarm the alarm from the computer interface, and if the alarm is armed, and a sensor is triggered (for example, magnets in a door are separated), then the alarm will sound, and to get the alarm to stop sounding you have to enter in a code into the GUI on the computer.

I think I can code the GUI fairly easily, but as far as hardware side of things I'm really not sure where to go. I don't have any experience with things like arduino. Unfortunetly for this assignmnt I have to write the code aspect in C# (Visual Studios .NET)

Any help is much appreciated :).

Thank you,

Buy one of the cheap USB I/O interfaces and start from that.