I'm thinking I want to hook up my PC to my TV pretty soon, and I thought it might be a neat idea to use my PC monitor for playing all of my old school systems. I've tried to search around and see if such a thing was possible, but I haven't had any luck finding a definite answer.
Do you know if there's some kind of cable or adapter that could help me?
Yes, get a TV to USB video capture dongle and instead of hooking up the TV end, plug in the RF. By "old school", I'd guess you're referring to systems using the RF AKA coaxial cable input - like the cord used for cable TV. Its a cool idea, so you can make your own videos of old games you're playing with screen capture software.