Try booting the HTPC up connected directly to a TV and then once booted up, disconnect from the TV and connect to the AVR. See if this works. You might also want to do a clean install. Upgrade installs do leave a lot of garbage around.
Originally Posted by mungee /t/1519099/win-8-1-x64-hates-my-avr-same-machine-running-win7-x64-worked-fined#post_24389580
Yesterday I got Win8.1 Pro x64 and all I did was "upgrade" the OS and now it hates my AVR (Pioneer VSX-1120K). The HTPC either does not boot (will perpectually restart) or the image on the screen is completely messed up.