Can you connec to the internet if you connect the CAT-5 directly from the modem to your PC? If not, right click my computer, select manage. Open the services and applications tab, then click services. Scroll down to DHCP client and DNS client services. Be sure they are set to automatic and they are started. If not, make those changes by double clicking and changing them. If you still can't get on the internet, click start, run and type cmd in the box. Then type ipconfig /all and hit enter. If you get an IP adress that starts with 169, you have a NIC problem. It should start with a 2 digit number most likely. If you get all zeros, you may have a NIC problem or a cabling problem.
If you CAN connect to the internet then you know your router is the culprit. Consult the maunal to see how to reset it to defaults, then connect the cables and reboot the modem, router and PC. Then do the cmd, ipconfig trick as above. Where it says default gateway, that is your router. Open a browser and type http://xxx.xxx.xxx.xxx
and hit enter (with the x's representing the IP address of your gateway). This will connect you to your router and you can then configure DHCP and other stuff.