Hi all.  

I'm a long time lurker, first time poster here.

I'm hoping someone can help me with a rather bizarre problem I'm encountering.  My HTPC has on-board video (an ATI4200), which I have disabled in BIOS, and am running a discrete ATI 4650.  Here's the problem.  Whenever I play any kind of video in media center or media player, I get a black screen.  I hear all the sounds, and on DVD's I can see the cursor on the title menus but not the menu itself (or the actual movie).  

Sounds like a straightforward codec issue, right?  Well, here's the kicker.  If I take out the 4650 and enable the 4200 on the motherboard, it works perfectly, without changing any driver or codec settings. I'm really stumped.

More information:  This setup previously worked fine.  I reformatted the HTPC this weekend for unrelated reasons (and, as it turned out, unnecessary reasons.) During the reinstall, I was getting a BSOD on IRQ_NOT_LESS_THAN, which I've come to associate with a misbehaving peripheral.  So, I pulled all extraneous peripherals (my hauppage tuner, the 4650, and a bluetooth dongle) to do the install, which then completed flawlessly.  After I had everything installed and working for a day or two, I reinstalled the 4650, which is where I ran into the black video issue.

system info:

The software environment is Windows 7-64 home premium on a clean install. Started with the Asus-ATI drivers on the motherboard's CD, then installed catalyst 9.12, followed by 10.1 when I noticed it was out.  
Other programs of note running are: Avast antivirus and Imon's remote suite.  I'm using the latest shark007 codec pack with the x64 supplement.  I first loaded the default settings, then used the "recommended" check-box in both, then modified my settings according to the "media center codec bible" thread found elsewhere on these forums.

Hardware:
Asus M4A785T-M/CSM (AMD 785 chipset) motherboard, AMD 2.6 Regor dual core CPU
Sapphire ATI HD4650 1gb video card
4GB DDR3 OCZ ram
Hauppage 1250 tuner
Antec MicroFusion 350 HTPC case
An older WD 320GB 7200rpm SATA Hard drive.  All my media is on my WHS though.

video output is via HDMI from either the onboard 4200 or the 4650.  Audio is via the optical toslink connector to my receiver, because sadly ,the Denon AVR 689's HDMI support does not include audio. (why denon, why?).

My working theory is that somehow, even though it's disabled in BIOS, video signals are still being routed to the 4200, even though the 4650 is handling desktop and 3d chores.  But where this switch may be enabled, I have no idea.