Gosh - sorry for not having understood!
I did what John suggested, then I detected another device with an exclamation mark, then again with the video adapter itself. Each time I removed them and rebooted. At last, when I "killed" the video, WinNT proposed me to reinstall it at reboot. I did it (refusing to go on internet, leaving it choosing alone locally). And now it works!!
I wish to thank you. Indeed I'd never have found my way alone on that one...
I just created a snapshot, in case...
Thanks again!
Hervé