I have a theory that you could pair the harmony ultimate with your computer by emulating a ps3 or wii remote. I think for mouse control you would have to set it up like a wii remote because the wii is the only device that lets you use the ultimate touch screen as a pointer. You would probably have to find software to interface with the computer in order to trick the ultimate into thinking the computer is actually a wii. On the mac side I think this can be done with remote buddy (software that lets you pair a regular ps3 or wiimote with your mac). I don't know of equivalent software for windows.
Trying this out for my 2011 mac mini (using remote buddy) is on my to do list. I can't help you out much for using Windows 8. For the mac side I would recommend remote buddy because you can do a lot of custom macros and other stuff using the built in IR sensor to the mini.