No video on 2 monitors after installing ATI Catalyst

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.

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

Any help appreciated.

TIA

Dan