Starting from scratch, as I'm not sure if your router has previously been in use. Power everything off, including the Cable Modem which should be left without power for two or three minutes.
Connect an ethernet cable between the router's WAN port and the Cable Modem's ethernet port. Connect a second cable between the PC's network port and one of the router's LAN ports. Power on the Cable Modem and wait for its four steady green lights (yours may be slightly different from my Motorola SB4200), then boot the router and wait for it to be fully up and running. Finally boot the PC.
If you need to configure the router, this is normally done by typing its IP address into your web browser. From the USR website, your router would appear to use the IP address 192.168.123.254 so type that into your browser. check in the manual in case there is a Password required. If you cannot access the router's Setup pages, try giving your PC's network adapter an IP address in the same Subnet as the router, for example 192.168.123.200 with Subnet Mask 255.255.255.0
Once the router has been configured, set the PC's network adapter TCP/IP settings back to getting both IP address and DNS server addresses automatically. You should then have network and internet access. Just plug the second PC into the router too and it should be online as well.