In Windows, just buy the Microsoft MCE IR receiver and whatever Harmony you want.
Or I'll ship ya a Dell MCE receiver (same one as MS but Dell branded) with remote, brand new from Dell, for $15. =)
Some newer computers don't have serial ports. That's really the best option, since as an RS232 device, you could write your own custom app to deal with the received IR codes. It's literally limitless in its capability.
For a plug/play USB option, well, I offered.