Run Knoppix from CD as I described, just to see whether it works at all. If it does, open a terminal window and type
Code:
ifconfig
Write down the output for eth0 or eth1 and compare it with the output you get when you run ifconfig from your hd install.

If you can see some sites in your browser, while others are not accessible, try lowering the MTU value for your NIC:

Code:
ifconfig eth0 mtu 1490
Keep decreasing it by 10 until you can connect. If this doesn't work, restore it to the initial value (usually 1500).