1. I am sure you already did that, but double check your PC is configurated as a DHCP client
2. Double check your machine can get online without the router
3. Make sure you use the right cables to connect from the modem to the router, and from the router to switch/hub, and from switch/hub to your pc. There are two kinds of cables looked very similar: crossover and strait thru. Read your router manual to see where should use what kind of cable
4. If you use a hub or switch, try to press the uplink button to see if it makes difference.

Hope this helps.