Open My Connections or Network Connections from Control pannel, then right click LAN Connection, go into properties click on Internet properties TCP/IP and click settings or properties and tick > Use following IP address then type: 192.168.0.1 to assign the PC 1 a IP, this may already be done, then click in subnet mask then this should auto configure. 255.255.255.0 or something simalar(Main PC with Printer and files or just decide it is PC1).
Do the same to PC 2 but give it IP address of 192.168.0.2 and if any more PC's go uo from there: 192.168.0.3 would be the next.
If in future you do want to share internet connection make sure in Advanced..,ON PC2 ONLY you add a default DNS Gateway, IP address 192.168.0.1 where 1 is the number of PC 1 shareing Internet conenction.
Hope this helps.
PS. Let me know how it goes on.