Robin Walkers NTL website answers many queries on sharing a cable modem
I am also on NTL 600k (which should now be 750) but just use a Linksys BEFSR41 cable/dsl router with switch to connect two PC's to the internet. They dont share files or the printer but have had no difficulty using either PC to connect to the net. Both use XP Home. This setup has now been in use for over 18 months with no problems. On connecting the router it was necessary to clone in the MAC address (physical address) of the main PC which was originally connected to the NTL server.
There is networking info on the Linksys website