The Control Computer generally resides in the shop somewhere near the CNC machine it is controlling. Control Computers dont have to be the top of the line, every bell and whistle type of computer. These are the workhorses. Save your money for a good design computer. You will save much more time having a fast computer for your designing.
A control computer doesnt have to be that powerful for a few good reasons. It doesnt have the workload. It takes G-Code and turns it into signals that it transmits to the Machine Controller. Turning G-Code into signals is not that labor intensive for a computer.
I have used multiple types of control computers. Everything from a 486 DOS running $30 special to a modern Dell with Windows XP. I have yet to try one with Vista. I will keep you posted. Generally I like to find a good used computer for a couple hundred bucks, wipe the hard drive and only load the control software onto the machine. That way you know what you have.
There is another good reason you dont want to spend a lot of money here. This computer will usually be located in the shop. Dirt will be located in your shop. Using logic here, dirt will be located in your...