If you don't already have the CAT cable then I would suggest buying a USB to CAT cable. No physical serial port or USB to serial adapter cable needed. The USB to CAT cable comes with a CD containing a driver that creates a virtual serial port that you can see when you open Device Manager on your PC.
Here's an example from Amazon: http://www.amazon.com/FT-100-FT-817-Chipset-Supports-Windows/dp/B0041LNISK
Ebay has numerous sources for even cheaper ones. Just search "CAT to USB" in the Electronics section.