Is there pci card, or USB device that can monitor things like a door sensor?

I want to record when my office door is open and closed (mostly to see if someone's opening it without my knowledge.)

I'd peek at those diy electronic magazines. They always create this stuff. Simple analog to digital devices.

Otherwise you have to use some stock automation device. Some home automation or an industrial device. Almost every automated machine has safety interlocks that interact to the computer (and to a hardware line too).

Security web sites may also have a cheap solution.

Just depends on how complex you want. The switch is easy. It is usually some magnetic or prox switch or such. Then it connects to some board that then either uses like a rs485 or ip or dedicated singnal to some computer. Then some program needs to run to detect it and log it.

Most often this sort of thing consists of a few simple parts like a magnet and relay on the door and frame and a simply set of wires to a serial or parallel port and some custom software to monitor it.

Developing a whole PCI card, with all the attendant card requirements, is often overkill to monitor a switch.

The X10 home automation stuff might be a pre-built solution. Do they have door monitors/alarms in the X10 world? The computer interface could get a little expensive.

How about a webcam pointed at the door with motion-sensing software on the computer? All of the required stuff ought to be low-cost, with the webcam the most expensive part. I believe motion-detecting software that starts recording the webcam is already out there for free.

arduino could be a solution here http://arduino.cc/

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Good call!

If you choose that route, some arduinos support "shields", pre-built add-on modules that plug on to the top of the arduino board. Here's a security sensor shield that allows you to monitor infrared sensors, switches, and reed switches:
http://shieldlist.org/freetronics/secsense