Can't say I'm much of a Windows expert, but this is somewhat of a weird occurence. I'm in front of a HP G61 Notebook laptop with Win7 that has connectivity problems. I mean that it's connected to the local network, Windows is absolutely convinced it can access the Internet, pinging arbitrary domains and IP addresses works, name resolution works too (and therefore must work UDP), but I can't connect to any website, or open MSN, or stuff like that. Even the Java Updater can't connect.
This is not a router issue, as other computers on the network can access the Internet. We also tried to join another wireless network, and found the same problems. Even with a wired connection, the same things happen.
My guess is that somehow, TCP is dead.