Questions about connecting a passive infra-red sensor to the USB port
You will need to use a device that has USB controller capability, or you can use an external chip. The actual device I would recommend depends on what kind of output the sensor has. Is this output On or off? If it's something so simple, I highly recommend using an ATTINY device like this. ATTiny simulates USB and works nicely without too much complexity.

However, if your IR sensor needs an ADC and some processing, you can use an MSP430, PIC, Cortex M3/M4 and many others depending on whether you need some processing of that signal.

Lets assume that you have the USB connectivity. In reality you need to choose a class for your USB device and for this I recommend CDC since it basically opens a COM port on the computer and you can send data as it arrives, then process it with any kind of program you like.