IR-detector questions

I assume you plan to use an IR photo diode, so you would start with a photodiode amplifier circuit. The most common method is to measure the photo current with an op amp (photo diode between inverting input and ground, resistor feedback). The non-inverting input can be connected to either a fixed voltage reference or to a matched photo diode kept in the dark but at the same temperature as the sensor and having the same bias voltage source (this should cancel the dark current error). The op amp voltage would be proportional to the photo current.

If you want to read this voltage into a computer, you will need an analog to digital converter with a computer interface. These aren't cheap. National Instruments has a good selection, and programming the acquisition is relatively simple with LabView.

If you are time-rich and cash-poor, but have access to a lot of parts, you could use an audio frequency chopper circuit to get your signal through the coupling capacitors in a sound card's microphone input. After that, you will need to write a program to detect and record the original signal (i. e., digitally "undo" the chopping operation).