ADSL USB Modem problem with K7S5A Pro

Hi there,
I have a problem with my BeWAN ADSL USB Modem after upgrading my PC
with a K7S5A Pro. All went (almost) well after installing the mother
board and rebooting my PC, the other peripherals are well working. My
ADSL modem is also recognized and running... for a few seconds. At
boot time, it is connecting and get ATM connection but just after it
stops working.
The modem was perfectly working will old mother board and is
perfectely working with an other PC.
I've heard that those MB had problem with USB alimentation. I suspect
that this is a cause of my problem. Does somebody already got in
trouble with those MB+ADSL modem ?
Anyway, I successed in using other USB peripheral.

The second hypothesis is that there is still some drivers for the old
MB on my hard disk (I did not reinstall Win98 after changing my MB, I
just booted...), that conflicts (each time I plug the modem it
reinstall it self the drivers although I did a complete - I guess -
uninstallation).
Thanks for any tips

Regards