I'm doing exactly that: AVR, PJ, HTPC and CirecTV using a refurbished Harmony 300i
(I'd paid under $20). It's cheap and works just like the original remote - YES you'll have to have a IR receiver on the PC but we're assuming you already have a working MCE remote!
The 300 is basic... no fancy screen, only one "macro" but a few of its buttons can be assigned functions and ALL of them can be reassigned to suit your taste inside their on-line programming application. I set the WatchTV macro to switch on the AVR to HDMI1, ON the PJ to HDMI2, ON the HTPC (which I'd reprogrammed the PowerON key for the HTPC device to Win-1 which fires off MediaPortal which is "pinned" to the taskbar). Sure, I could change it to PowerON the AVR and HDMI2 then do nothing on the Satellite receiver since it's ON all the time anyway, and be ready to watch satellite!