Hey what's up.
How is your project coming along? Well, I'm no electronics engineer, but projects of this nature fascinate me. I'm always thinking about interfacing things with my computers. Anyway, I figure you are talking about standard motion detectors, the variety that have flood lights that can be programmed to turn on when motion is detected, right?
If you're handy with electronics, then find a schematic for the device or use a micrometer to find what wire or wires are being used by the motion detector to send a trigger for the light to come on. Once that wire(s) is found, you should be able to either splice into or re-route the wire to a serial port connector. You can find serial port connectors at radio shack or computer shops. You could also use a USB port, either way you'll have to find a diagram explaing what pins can be used for what. The electrical trigger could be used to activate a counter. Take that tapped wire and connect it to a recieve pin that will be able to take the signal as a bit. Instead of turning the light on, you can write your program so that this single signal will count and display the number of motion triggers. Just relay that signal to a pin on a usb or serial port and have your software count how many times the unit detected motion.
You should try and contact a motion detector manufacturing company. You may be lucky enough to find a friendly engineer that will have a good amount of information for you. I think writing the program will be the fun part. A valuable tool is the Nuts & Volts magazine. Write to them, they'll send you a few free magazines. They have plenty of project like this. You can find them on the web at : http://www.nutsvolts.com/
(no i'm not an employee.)
You could also use one of those line of sight devices that ring "ding dong" when the laser or light beam is broken. This thing is usually found inside of convenience stores or liquor stores. If you could interface the device to a computer with a serial or usb port. Again, you would have to get some schematics and do some splicing.
Lastly, you might want to find an electronics engineering student or graduate from a school like ITT or your local university. They are involved with wiring devices like you need. You could possibly do this project together with a student. You'd be surprised at how easily accessible help is.
Hope this was a little bit of help. Good luck and let me know how your project turned out.