thank you folks for your help,
my win XP install is "clean", a full format after
my change of Motherboard.
i have Direct x 8.1 installed with the catalyst drivers going ok.
i have found the preference file in the CMBB folder,i deleted it and had it recreated by CMBB, it seems ok.
now CMBB crashes completely my computer,
my PC loosing completely its power...
i think the power of my PC (300w) is not powerful
and stable enough,causing CPU errors or power shutdown.
i'll change it for an AMD certified 400w power.
wait and see...
i'll let you know how it went, thank you again.