You can do it, but as
@TLS Guy said, you don't put amps in the same case as a PC. They have to be isolated. You could do what
@TLS Guy did and use a DAW type HTPC which would get you a very high end audio interface that has excellent AD/DA converters that will sound excellent, but the issue isn't whether or not it will sound good, it's whether or not you can get features like the Dolby or DTS upmixer found in current AVR and Pre/Pro. You can't. Simple as that. Not only that, but even the PowerDVD software linked above (which I've used for many years and is one of the last of it's kind) can't decode and output any of the object based formats. That's what you're missing.
If you want to do a straight 5.1 or 7.1 system, that is easily done with a PC. I've done it. I used a PC sound card capable of 7.1 output and plugged everything into external amps. It worked fine for a while, but there is a lot missing from a set up like that from simply using a regular AVR or Pre/Pro.
Most of us just have a purpose built HTPC that plugs into an AVR or Pre/Pro and external amplification that way we get ALL the bells and whistles. The PC is a source for the rest of the system, not the ENTIRE system.