You can buy expansion cards that will add serial ports to your computer. They're not expensive, so that's probably your best bet assuming you're going to do this with a desktop PC (you didn't say).
I have some old data aquisition software that I bought a long time ago, and it only uses com ports to communicate to external devices. But all of my devices are using usb connections, and I only have usb ports on my computer. I need to somehow make the usb port show up as a com port so that my data aquisition software can collect the data from my devices. Is there some free software out there that makes usb connections show up as com ports?