Hi tup...
From my own personal experience, running DSL from a USB port can be touch and go, at least with Windows 9x, I can't verify with NT/2000/XP.
My best suggestion to you would be to buy a network card and cable (or wireless setup) and run your DSL through that route. Much less hassle. Make sure your DSL modem supports wireless connections before buying any wireless equipment, if that is your preference in network connections.
In what manner did you turn off Avast's services?
Best Regards...