Do you have Vista? I ask b/c I am a laptop technician for a college and lately we've seen several Vista computers that for whatever reason no longer access the Internet on ANY network. They all get on AIM, iTunes store, MSN, their antivirus updates, Windows Updates run, etc, but you cannot browse in IE or FireFox.
The first thing I try is a System Restore back to when they can confirm they were online. This has not worked more than it has, but it's something to try. If it doesn't work, then I delete the network adapters in the Device Manager. If you have Vista, they will instantly reinstall themselves. Then I open a command prompt and do an ipconfig /release and ipconfig /renew. What we are seeing is that the computers will release and renew the correct IP addresses and we can see these computers on our network, but there's something wrong with the TCP/IP that's simply not allowing them to browse. All of my "guinea pig" computers were sent home and even after the networking guys did hours of research, they said to reinstall Vista.
Now, I'm supposed to find a new guinea pig computer and run this in a command line and see what happens:
netsh int tcp set global autotuninglevel=disabled