Are you using a wired or wireless connection?
Have you installed all the drivers for your motherboard and network cards?
Have you made sure you have a cable plugged in? That lights are blinking on that cable?
If you go to network connections, are you set to get an address automatically?
If you go to start, run, cmd...and at the command prompt type ipconfig, what does it say? If it's 169.x.x.x, can your type ipconfig /release. When that is done, type ipconfig /renew.
Does your router or something else hand out via DHCP?
If you know the network range of IPs, can you manually enter one in the network? What happens then?
We may need a little more help to get you going with this...some more information would be great.