Just in the past few days I've seen several machines that can't get anywhere on the internet, through either Internet Explorer or Firefox.
The thing in common was AVG 8.0 so I did some investigating. Turns out, the Webshield / Surf Protection had become corrupt, and was preventing the browser from going to any web site.
To make a long story short, you simply need to obtain the latest installer package for AVG 8.0, and install it using the "Repair" option.
The install package can be downloaded from here. Download the file"AVG 8.0 for Windows" (avg_ipw_stf_all_8_1xxa1xxx.exe). The latest file should always be available here.
So, if you get a machine that has an IP address, can ping, can do DNS lookups, but still can't get to any web sites… check for this problem.