Running Windows XP Media Center Edition SP2 on a computer with two
video cards.  I was experimenting with the display settings when I made
the secondary (PCI) video card the default display and set its refresh
rate too high (so that it would not display on the monitor at all).

I was able to get back to using the primary (AGP) video card as the
display by booting in Safe Mode and disabling the secondary (PCI) video
card in the Device Manager.  At least I can see the screen now, but I
can't get the secondary (PCI) video card back.  When I try to re-enable
it in the Device Manager, the screen goes black (blind again) and I
think it must be hanging the system, too, since no combination of keys
or mouse clicks do anything.

I think if I can manage to reset the secondary video card's display
settings prior to re-enabling it, I'll be all set, but I can't do that
using the Display Properties window, because with the secondary video
card disabled, it doesn't show up at all in the Display Properties.

Is there anywhere I can find these settings?  In the registry, perhaps?

I understand this is a fairly complicated problem and I thank everyone
for their help.