The best option is a router. In that case, the modem would plug into the router and each pc would also plug into each router. The router would get an IP from your ISP and the router would provide internal IPs to both of your PCs.
Which PC is the modem currently hooked up to? If hooked up via USB to PC1 then I believe you could network PC2 to PC1 (both need NIC) and then configure PC1 to share the connection via ICS, for example. This is not the best option because PC1 would need to be on at all times for PC2 to connect.
Which PC is the modem currently hooked up to? If hooked up via USB to PC1 then I believe you could network PC2 to PC1 (both need NIC) and then configure PC1 to share the connection via ICS, for example. This is not the best option because PC1 would need to be on at all times for PC2 to connect.