Help with no video situation

I built a pc about 2 years ago including an Asus P5B mobo & a Radeon
X800GTO vid card. I use this to drive 2 monitors, a desktop LCD & a
wall mounted plasma TV in another room, the 1st by VGA, the 2nd by DVI/
HDMI cable.

I recently installed XP pro on one of the 6 hdd\'s in this system, to
dual-boot with the existing XP home. Initially, this new OS would only
appear on the LCD. So I dl\'d a copy of the latest ATI software
(Catalyst) to go with the vid card for this 2nd OS. After installing
Catalyst, the new OS only appeared on the plasma. Referring to screen
shots of the various ATI settings which work for me on the original, I
began making settings in Catalyst on this new OS. Rebooted, NEITHER
monitor shows the OS after the initial XP logo, the screens just go
black & indicate they are not getting a vid signal! How can I get into
the settings, if I can't SEE the bloody OS! ;-) Is there a way I can
activate safe mode during boot? I tried pressing F8 during boot, if
it's pulling up the "safe mode/last known good config" option, I can't
see that either, because the monitors remain black. I am still able to
use & access the hdd/files of this 2nd OS by booting into the original
OS, tried changing the name of the ATI directory, thinking if this
could not be found by the new OS, perhaps it would default to some
usable config, but no such luck. Maybe there's some registry edit I
could do on the new OS from WE while in the old OS?

I should stress again that both monitors continue to work perfectly on
the original OS.

Any help appreciated.

TIA

Dan