The problem with the above answers is YOU HAVE TO CONFIGURE TCP FOR YOURSELF.
As long as you only want ComputerA <--> ComputerB and neither to have Internet access, you do not need an ISP connection. You apparently do as you are accessing Techspot.
Your ISP connection has a Modem and it may have an internal router.
isp === modem/router --- PC
If there are more than one slot for the PC(s) to use, just connection the second system to the ISP device.
If there is only one slot, then you need:
isp === modem/router --- switch --- multiple PCs
Before we make assumptions, please tell us the Make/Model of the device connected to your ISP.