First, some relevant specs:
EPoX AD580XR motherboard (Socket AM2, AMD 580X chipset)

The motherboard sports 3 PCI slots. Problem is, I can't get either:

Creative X-Fi XtremeGamer
ATI Theater Pro 550

detected by Vista Business x86. As in, no driver prompt and installers fail because they'd like the hardware to be installed. At the same time, SiSoft Sandra sees the cards in its probes. I've reseated them several times, tried every combination of cards and slots, hit the failsafe bios settings, but no dice. In addition, I've done a few searches through the MS KB, but with no effect.

EPoX doesn't have anything for me either.