Why, why, why?
Seriously, every place I've at with a serious industrial control system uses ancient hardware and software (like modems and hyperterminal), because that's what the industrial machinery's built to communicate with, and it's cheaper to keep using a $5 PCI modem than re-engineer everything to use Ethernet.
So why rock the boat? In a previous life a few years back, we monitored a 9 figure datacenter environment with a Windows 95 whitebox PC, because that's what all the industrial systems were designed to communicate with. Had a bunch of identical spares on the shelf, hourly backups of the system, and when it failed, we popped a new one in its place and dropped an image on it. (Only happened once in ~12 years, which was pretty impressive.)
What you're doing here is trying to create a solution for a problem that doesn't exist. Leave the thing running Windows XP and a modem instead of trying to spend thousands of dollars to do the same thing that you're currently doing for free.
EDIT: Still a bad idea, but if you don't have a choice, I think this is what you're looking for.
The Null-modem emulator (com0com) is a kernel-mode virtual serial port driver for Windows. You can create an unlimited number of virtual COM port pairs and use any pair to connect one COM port based application to another. The HUB for communications (hub4com) allows to receive data and signals from one COM or TCP port, modify and send it to a number of other COM or TCP ports and vice versa.
Still a better/easier idea might be to just buy a PCI modem that comes with Windows 7 drivers, such as this one, this one or countless others. Good luck with that mess, by the way.