I took some time off from the problem for Labor Day weekend, so now I'm back at it. Here's a rundown of what I tried:
(1) I rebooted after removing all ATI files and graphics drivers using "Driver Cleaner .NET".
(2) On reboot, Windows automatically installed it's default VGA driver titled "Standard VGA Graphics Adapter" (driver version 6.1.7600.16385 by Microsoft).
(3) I run "10-1_vista64_win7_64_dd.exe" (the display driver <only> executable for driver version 10.1; I'm trying an older driver in the hopes something will work
(4) Catalyst Install Manager Installation Report says Success was achieved for "ATI Catalyst Install Manager" installation and "ATI Display Driver" installation.
(5) On reboot, system POSTs, loads the BIOS, tries to load Windows, but fails part way through, going to a Blue Screen and then rebooting.
(6) On reboot, inspection of the Device Manager's Display Adapters states "Windows has stopped this device because it has reported problems. (Code 43)". Windows still recognizes the card as an "ATI Radeon HD 5900 Series" card.