hi jnthn
As already posted, you require two pairs of IR transmitters and receivers, aligned at right angles to the objects path, spaced at some distance apart along the path.
If your object is carried past your IR transmitter/receiver pairs a at 'moderate' speed,[let us know, in say mtr/sec].
You might consider using the PC's parallel port input pins to get the IR receiver TTL signal into the PC.
The PC program would poll the port pins, get the TX/RX pair #1 signal as a start, read the PC's internal Timer at T1, then the TX/RX #2 signal to stop the timer at T2.
By setting the TX/RX pairs about 1 metre apart, along the path of the object, the 'speed' of the object will be directly proportional to T2-T1.
If you need a sketch let us know.
Regards
EricG