Interestingly enough, I have a very similar problem with my Gigabyte GA-MA790FXT-UD5P-based desktop. Though in the directory you posted, I have viamrx64.inf and viprt.inf rather than the files you mentioned. It's also a Realtek HD onboard sound chip though.
I'm running XP64, and the BSOD comes up probably 50% of the time I restart the computer. The error mentions portcls.sys but doesn't say that it's missing.
Also, even though sound works, every time Windows logs me in and attempts to play the startup sound, I just get an "invalid audio stream" or something like that in an error dialog from Realtek HD. However, when I go to "Sounds and Audio Devices" and test the startup sound, it plays just fine.
I'm a bit disappointed in Gigabyte's audio drivers. It's kinda cool that I can plug my stereo system in however I want and the Gigabyte driver will ask me what I just plugged in (headphone, mic, left channel, right channel, surround, subwoofer, etc.) and dynamically assign the audio ports that way, but this BSOD problem and the startup sound issue does not reflect well on their quality control system. Also, it does get kinda annoying having to select "headphones" every time I plug the headphones into the headphones jack--I wish the Gigabyte drivers allowed you to "fix" the device assignment for the front audio jacks.