I disabled onboard in the BIOS with no effect. I don't use HD Audio on Video Card but it is disabled in Playback Devices.Do you have the onboard disabled in bios? If "no" then disable it in bios, if "yes" then maybe enable it in bios but disable it in device manager? How about the HD audio off of the video card? Do you use it? If not, again try disabling it in device manager.
Make sure you've uninstalled all drivers for the onboard audio. (ccleaner is supposed to be safe again.... ).
If it were my PC I"d focus on possible hardware conflicts first.
I use onboard sound for my Roccat Kave Headphones and Audigy RX for my 5.1 speakers.