You will have to buy two 2N4041 or any general purpose NPN transistor (BC548, for example), a 10 nF capacitor (a.k.a. 0.01 uF), a 4K7 resistor (yellow, purple, red), a 47 K resistor (yellow, purple, orange), a 15K resistor (brown, green, orange), a 22 ohm resistor (red, red, black) and a 1 K resistor (brown, black, red) (all of them 1/8 W or 1/4 W). As for the infrared LED and the infrared photo-diode, use Radio Shack 276-142. All these components can be bought at Radio Shack.
Figure 4: Infrared device schematics.
When assembling the circuit, the only special care you will need is to make the transmitter (LED) and receiver (photo-diode) become perfectly aligned, side by side, as shown in Figure 1.
After assembling the circuit, you will have to connect it to the motherboard IR connector. Notice that we have marked four points in the circuit: Vcc, Gnd, TX and RX. These points should be connected to the corresponding points in the motherboard IR connector.
It is also important that you configure the IR interface to operate in full-duplex mode at the motherboard setup to achieve higher performance.
After the device is connected, it should be automatically recognized by Windows, and it will show an infrared icon on the task bar near the clock (lower right corner). As soon as you approach your cellphone, handheld or notebook infrared sensor to the device you build, Windows will alert you that it found another computer nearby, allowing you to transfer files between the device and your PC.