Is there an adapter to plug an audio/video cable into a usb port on a pc?

I've seen something like it but I'm not sure if it does what I think it does. I want to plug my xbox 360 into my old imac in my room and I need an audio/video -usb adapter to do this. plus there is no way in hell I'm going to pay $40 for this, it should be under $15. the only ones I can find say audio/video grabber or something weird for comeras or vcrs or something. tell me if this simple adapter exists and what it's called and where I can find it. thank you