If you add a PCI device and Windows NT hangs, the HAL has assigned the new PCI device an I/O port address or memory address used by the existing PCI devices. Remove the new PCI card and reboot. If the system is back to normal, you can use the /PCILOCK switch to boot.ini to cause the HAL lock these used resources to these PCI devices. This was added in SP4 and Microsoft's HAL.DLL, HALMPS.DLL, or HALAPIC.DLL will respond to /PCILOCK directive. Using the parameter is a good safe-guard anyway since ntdetect.com has been known to dynamically switch resources on a working system to a set that are overlapping and thus conflicting.

If the system does not hang but a new PCI card has conflicts, the best tack is to place the new PCI device in a different slot (if another if free) and if not, to experiment with the order of cards in the PCI slots. Not very elegant. I have seen PCI card combinations that I simply could not get to work together. Some feature-packed sound cards and video cards have been this way.

Once you get the set to work, put the /PCILOCK parameter in place.