Connecting pc to real world

I'm in the early stages of planning a HA system for a house we're going to build in about 2 years. I'm a DIY kinda guy and am planning to use a touchscreen pc to control security, heating/cooling, solar water heating, landscape watering etc. We're not real big on audio/video, so probably won't have any of that integrated into the system. Writing the code to control everything is within my abilities. My question is how does the pc communicate with the real world. The touch screen that I'll be using has a USB port and a serial port. How exactly do I connect the various sensors, both contact closure and analog to the pc and how do I connect the pc to the various pumps, valves and external systems to be controlled? I presume some sort of I/O device that plugs into either the serial or USB port and provides X number of 'channels', but I've googled around and haven't come up with anything. I'm looking at perhaps 100 input and half that number of output channels. How do you guys do it? TIA Paul