My lowest end systems is very similar to yours, and a few steps down. Same proc and audio (Asus N8 with a 64X2 4200+ 1Gb ram, X-Fi sound card....) and Showtime plays HD and BR just fine along with HVD, Win-HD downloads and everything else I throw at it. Vista just takes a LOT of patching of everything you plug into it. One thing of note, there is not DIRECT x16 support in vista. 16-bit software (including most hardware packages released for XP and the early vista driver kits) run in a ghost, or virtual support mode under the 32-bit version of the OS.
Noted here:
High Definition Audio Controller
-----------------------------
Driver
Description : High Definition Audio Controller
Company : Microsoft
Version : 6.0.6001.18000
Date : 6-21-2006
File Name : hdaudbus.inf
You're using Microsoft's built in controller file, try downloading your sound card's own driver set directly from the web site and switching from the 97x/98x to the HD package found here if the chip-set will let you:
http://www.realtek.com.tw/downloads...=24&Level=4&Conn=3&DownTypeID=3&GetDown=false
When you go to install turn off Vista's safety and protection features. That will keep the installed driver from reverting back to the default. Some revisions of the 97xHD and 98xHD chipset will take the full HD drivers and some won't, but it doesn't hurt to try.
Also, the Hauppauge input boards are know to have issues with Vista. If switching/updating the audio software doesn't fix your problem, try updating that software as well.
You may consider heading over to freecodecs.com and downloading the Klite Mega codec package (and set it to install "lots of stuff, despite the so-called issues some report, I've never had a problem with it), as well as the Vista HD codec package. Passing things through FFDS speeds up the processing of AV and helped for me.
If all that doesn't work, try playing back something with the Hauppauge WinTV card removed. That's my guess as to what the problem is.