AirPort Issues
 

I have a SBC Yahoo 2Wire System for DSL in my house. We run two Windows XP computers, the main computer has a modem and the other computer has a wireless adapter.

I just bought a Macbook and I can connect to the network, but it says I'm not "connected via PPPoE" which means I can't connect to the internet. Is there a way around this, or am I just screwed?

You have the 2Wire HomePortal system? The HomePortal establishes the PPPoE connection itself, and then acts as a standard wireless router on top of that. Clients connected to the HomePortal do not need to establish PPPoE connections.

Well that's good to know. But is there a way to explain why I still can't connect to the internet?

You should check the Network preferences to see what kind of IP information you are getting back from the Wireless Router when you connect with AirPort. Or, if you are familiar with the Terminal application, run the command sudo ipconfig getpacket en1. That will print out the DHCP reply packet (I assume you are using DHCP to configure wireless clients) that Airport got back from your wireless router.