I struggled for ages with a Conexant chipset based USB modem someone gave me in both 98SE and XP. Tried driver X and driver Y, hacked the driver files and Lord knows what else.
Eventually got it working in 98SE, but it kept dropping the line.
Binned it and bought a US Robotics - worked first shot.
Now I know why they gave it to me!
Seriously, web forums are littered with startk raving mad people who have tried to get Conexant chipset based softmodems to work with properly with anything.
If at first you don't succeed - give up.....