1.You can simply use a Ethernet 10base-T hub to share the Internet connection....with all PCs.
If you using the broadband is 'Always On' mode / Router Mode.
Just Insert your modem's Ethernet cable into the Hub's UPLINK port and PC's are other remaining port,for setup correctly follow the Hub's Manual.
2.If you using PPPOE Mode / Dial-Up mode then,
open network connections.....right click the connection--->properties--->Advanced---->Under ICS---->Check the "Allow Network other network user.......Connection".
You can also check the other 2 option if you need though...
ok----> Apply.
In this case you will need to On the PC where the modem physically connected...to get Internet to the other PCs.Both method make sure all the PC use the same class IP address and same sunet mask.
as expl---> 1st PC's IP:-192.168.1.10, 2nd PC's IP:- 192.168.1.11 , 3rd PC's :----12, 4th PC's----13,Subnet must be:- 255.255.255.0 for all PCs
Default Gateway :- 192.168.1.1
Prefered DNS:- Consult your ISP manual's page
Addtional DNS:- ------Same Above Line----------