|Easiest and most effective way for now would be to turn off DHCP on the router and use cables to connect the laptop and PC to the LAN ports of the router (not the WAN port). This essentially dummies down the use of the router so it's really only acting like a hub.|
Then on the laptop go to the Network Connections window where you see the icons for your network adapters, right click on the ATT adapter and choose properties, then under Advanced enable Internet Connection Sharing. If a drop box exists under "Home Networking Connection", choose your Local Area Connection or Ethernet port and apply the changes.
Here is a link with more info on enabling Internet Connection sharing...
What this does is to enable your laptop to share the ATT internet connection through the LAN port to the desktop. Your laptop becomes the Internet router. Get that working, then you can try connecting wirelessly to the router instead to give your laptop more mobiltiy (and the Internet Connection Sharing should still work, but you'll need to change the drop down selection on the Laptop to the Wireless Adapter).
Assume that I already did an Internet search.