I am using a USB Interface Cable for FMS that works good with a Hitec Eclipse Xmitter buddy-port and USB port on Windows XP.
The device simulates coming in on COM3 or COM4, so you don't need an actual COM port.
The site is a little funky, but he has a cable to fit just about any Xmitter(including square or round):

http://www.tti-us.com/sim/prod-USB.html