May sure the two computers are under the same Workgroup Name. If you are running XP, re-run it and go to additional tasks and configure your network. Contact me through firstname.lastname@example.org for more
The problem is not with Windows or have anything to do with a workgroup name, the problem is you only have 1 IP address and you need NAT and DHCP to allow the other PC's to access the internet. You want to buy a standard home router and connect that to your modem. These home routers include a built in 4 port switch as well as include DHCP services so all PCs can get on the internet.
This would also be much more secure because your current setup is not using NAT and most likely not using any firewall features either.