Connecting pc to real world

["Paul in Redland" <paulinredland@xxxxxxx>]

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