LAN speed weirdo

Hi,
I have just acquired a new client with a strange problem.
3 computers networked p2p. 1 computer (called server) running XP SP2, the other two XP Home unsure as to release (though probably SP2)
The server has cable internet via Motorola Surfboard SB 5100 modem, connected via USB port. LAN separate connection via Intel (?) chips.
Now the funny bit.
When the LAN is set to 10Mbps internet works fine, as does the LAN (though slow).
Change LAN speed to 100Mbps and no internet connection, though LAN works OK. Ipconfig /all gives same results in both modes.
I need to get the LAN to work at 100 Mbps and the internetand am totally bamboozled. Any ideas?