Setting up ICS breaks Internet connection

I have a Sagem DSL modem supplied by Tiscali connected to my computer
by USB. I have Win XP [home/pro/service pack level 3]. I want to use
Internet Connection Sharing to connect another computer. When I run the
Network Setup Wizard to setup ICS, everything works OK until I reboot.
At that point, my Internet connection is gone and I get error 720. The
Internet connection survives a reboot if I don't setup ICS."

I have run netsh int ip c:\resetlog.txt and netsh winsock reset, but I
still have the original problem.

In addition, if I use a different computer, which has Win XP
[home/pro/service pack level 3] to connect to the Sagem modem, I can setup
ICS on that computer and have no problems after rebooting.

This is proving to be an irritatingly intractable problem.