What is the error when you try and go to a web site? Is it a 404, timeout, or something else?
I'm wondering what the connection quality is on our wireless setup. you might be able to ping something but that doesn't necessarily mean that the connection is of a good enough quality to surf the web. Try doing 50 pings or so and see what the latency in 'ms' is. Do a 'pathping' on a few web sites and see what the lost / sent percentages are.
You may want to try making a connection using a few other protocols like ftp, telnet, ssh, etc. I'd try and go to an ftp site through the command prompt and see if you can make a connection and sustain a download. If you can make a sustained connection with decent speed through a different protocol, then I would guess that the problem could be narrowed down to things like the http protocol or the browser's configuration.
Can you try to connect any other wifi device to the network and see if it can access sites?
Let us know what happens.
EDIT: What is the wireless signal's strength? See if your wireless connection utility can tell you things like signal strength and signal to noise ration (SNR). If not, NetStumbler should be able to do it.
