Why doesn't your computer have 7.1, most of the time its standard built in with the motherboard.
My motherboard z97 was released in 2014 and it states clearly - 7.1 CH HD Audio (Realtek ALC1150 Audio Codec), Supports Purity Sound™ 2 & DTS Connect
What are you connecting from the PC to the AVR and does your AVR support 7.1?