Found the problem. I identified - in the registery - the base system device as being the "TI UltraMedia Firmware loader device".
Reloaded the TI driver and then the Dell driver set.
Now everything works.
My computer is not Dell, but I have encountered the same problem. (Toshiba Satellite 2410, in Windows XP system)
My computer find an unknow device as base system device, and I can't find the driver about it.
I don't know how to fix the Registry Editor.
So can you do me a favor, step by step. Thank you for your help.