Tibbo, Lantronix, Netburner and Wiznet do such modules. With the Wiznet I had some issues. Maybe because of the Power supply not supplying enough current. Never managed to make it work. Lantronix is my favorite though most expensive I suppose.

Anyway, they all are in the logic of Serial to Ethernet and then your PC will run a piece of software to read the Ethernet and convert it to Virtual Com Port. Then your old software can just read the virtual serial port as if it was a real Serial data arriving from your Serial device connected locally.

Ioannis