So I just upgraded from XP Pro to Vista (Home Basic 32-bit), everything worked fine under XP. My graphics card wouldn't output until Windows loaded, but the onboard vga would output until then. Since I have installed Vista only the onboard vga adapter works. The ATI card is being correctly identified and Windows attempts to install the dirvers for it; but then it says "Incompatible display adapter has been disabled. At least one display adapter on the system has been disabled because its driver is not compatible with the driver for the VGA adapter." Then the device manager shows the onboard VGA adapter as working correctly and the ATI as having suffered an error during install.
The System is a homemade one:
Intel Core2 Duo E6550 at 2.33
ASRock ConRoe 1333 - D667 R1.0 (the onboard VGA adapter is Intel 82945G Express)
4Gb of Ram dual-channel
and of course the Radeon HD 2600 XT in a PCIe slot
I have tried multiple different versions of the ATI drivers, driver package, and software suite, I just updated the VGA adapter, and Microsoft update is up-to-date.