The best part about using a PC is that they are infinately customizable. With a USB IR receiver and the proper software you can make them do anything. By default the Roamio remote uses RF for all the main TiVo buttons, but still uses IR for Power, Volume, Input and Mute. You can switch all the buttons to IR by holding TiVo+Red C for a few seconds. (The light blinks red in IR mode or amber in RF mode)
There is a popular product called flirc that allows you to learn any remote command and then map it to a function in MCE. You could just assign the volume buttons to some popular brand AV receiver and then use it to map thise commands to the internal volume of Windows if that's what you wanted.