Your computer is working fine - the fact that it's exposing Dolby Digital as supported tells us that. Basic Windows drivers have supported 7 channel PCM over HDMI for what seems like eons. It's how I get away with using such an old receiver (Denon AVR-2807): I let my media center PC decode the bitstream and hands it off to the receiver as 5 channel PCM, with the receiver only doing delay/room corrections.
Here's the trouble:
wincel wrote:The Vizio is supporting HDMI in and out 5.1 etc.
Well, it kind of does. It will indeed output a DD/DTS bitstream from its own internal sources, and it will accept a similar DD bitstream coming in, but here's the catch: PCM processing in the TV is limited to 2 channels, so if at any point it gets processed, it exposes only two channels as available, and this is the intended behavior.
There are two choices: one might work, the other definitely will. The first is to change your audio output mode on the TV from PCM to bitstream - this might work. Of course, given it's a Visio, it may just always expose two channel as its only PCM input option regardless of any other setting - their documentation on it, and the rest of their products, sucks (I might be spoiled in that regard though).
The guaranteed method is to run your HDMI into the audio system, then out to the TV from there. If this isn't practical, then using a TOSLINK cable or other separate digital audio output is your best bet - PC sounds will still be in stereo, but whenever you have 5.1 source content it will simply stream it to your external decoder.
Good luck and let us know how it goes!