I have a mac book that up until Monday, was connecting to the internet through my Westel router. Now the computer connects to the network, but will NOT connect to the internet. My PC and PS3 connect fine.

The mac book says, "self assigned IP may not be able to connect to internet." I cannot figure this out. I have deleted the key chain and re-entered. Still connects to the network, but not the internet. HELP PLEASE!

Go to System Pref > Network.
Select network card that is being used. Most likely Ethernet.
Near the Middle, towards the top, is Configure with a drop down box to the right. Select Use DHCP.
Click apply and that should do it.

Moviedude is right, but, I've found that that changing it from DHCP to another opertion like BootP and Apply it and then change it back to DHCP and apply. I don't know why but this works for me when I need it.