You would think in 2014, a corporation like Kenwood would have good enough programmers to create a software package to program an amateur radio via a modern computer without a USB to serial converter, but I guess not. And if they had to use a serial converter, the program would be compatible with some of the major manufactures of USB to serial converters!
After obtaining a Belkin USB to serial dongle, and installing the drivers, I've had no luck. All the COM ports on the Kenwood MCP programming software would be blanked out. Installing on multiple OS still didn't register any open COM ports on the MCP software.
After doing extensive research on the net, I found someone who had luck with a Plugable USB to serial dongle. This dongle has a Prolific chipset, which seems to be the key.
I received the dongle today via Amazon, loaded the drivers from the web on my XP machine, plugged in the adapter and connected my Kenwood radio. After launching the MCP programming software, it was able to see the adapter on COM1, COM3 and COM6. After choosing trial and error, it settled on COM6 and could read the TM-71A. Success!
Also, don't purchase the programming cable from Kenwood for $50 bucks. It's a generic straight through cable that uses DB9 on one side and mini-DIN on the other. Hosa makes a cable that works perfectly for this type of application.
Here are the parts/software I used.
Kenwood MCP Software Download Page
Plugable USB to DB9 Serial Dongle (Rev D)
Hosa DBK-110 10 Foot Synthesizer Controller Cable, 8-pin Mini-DIN to DE9