code 12 with geforce 6600 GT added to HP Compaq dc7600 cmt

1.Configuration:
HP Compaq dc7600 cmt desktop with onboard graphics Intel card.
Added Nvidia Geforce 6600 GT PCI Express card.

2. Configuration worked perfectly on Windows XP Pro

3. Code 12 error for Geforce 6600 graphic card under Windows Vista Bussiness

Did anyone find a solution yet?

What I have tried before:
a) Vista Business is a clean install after reformatting the hard drive
b) Motherboard has been upgraded to the LATEST BIOS available form HP. Code
12 still there.
c) It is not possible to disable onboard graphics from BIOS. BIOS disables
it automatically as soon as other graphic card is installed.
d) If I take out the Geforce 6600, Vista works flawlessly. Recognizes the
onboard graphics, Aero starts to work, no errors anymore. When I reenter the
Geforce 6600, code 12 comes back.
e) Tried to disable everything that is possible to disable in BIOS and that
could take away resources from Geforce (onboard network, onboard sound, USB
etc). Code 12 still there.
f) Tried to disable everything that is possible to disable in Device
Manager. Code 12 still there.
g) No apparent resource conflict in Device Manager. That is, it is not
discernible, what takes away resources from Geforce. I presume it is some
conflict with the onboard graphics, that Vista cannot take of its mind.
h) Code 12 is there after clean Vista install with NO Windows Updates
applied yet.
Code 12 is there after ALL Windows Update applied.
Code 12 is there after ALL HP updates applied.

I thank for any suggestions in advance.
I will be glad to answer any questions.

Sincerely,
Ambroz