I have exactly the same problem, I have a DSL USB modem too. I'm using w2k.
The problem only appears if I use P2P programs (like Shareaza or BitSpirit) that create a large ammount of connections. I'm not using MSN.
When that happens I can still control the modem using the driver interface (stop, restart or run tests...), so the modem is still working.
I tryed to search on google, I found a lot of people having the same problem than us but with no solution. That must be another bug on Microsoft side.
People who have XP may solve the problem installing SP2...
my google search:
If one of you found a solution PLEASE tell me