since a motion detector has only 2 states...ie detected or not detected...try using parallel port...or a serial port...a parallel port is better...cos u can use it in c as a ports of a microcontroller......but be careful ....if a higher voltage if applied(may be >5V) might burn ur motherboard (this hapnd to me)....
Look over www.abcelectronique.com but it's in frensh, but you can make a shema by yourself just be carful you must be sure about the matching impedance between the port and the device and you can use a voltage decreaser made by resistance for the mother board to be safe even if the voltage extends 5V.