It should simply be a matter of:
1) Turn everything off.
2) Connect the modem to the router's WAN port (use the ethernet cable ntl supplied). Connect the PCs to the router's LAN ports (use straight-through ethernet cables).
3) Turn on the modem, & let it boot. Turn on the router & let it boot. Then turn on the PCs.
The router's internal DHCP server needs to be on (is probably on by default) & the TCP/IP settings on the PC's network cards need to be set to Obtain IP/DNS automatically (see the second link in 50420's reply).
Have a read of
http://www.cableforum.co.uk/board/showthread.php?t=8269