I had a call from a friend earlier this week with exactly the same problem - she could do most net-related things, but not browse. Ipconfig /all showed a connection, but with a very strange IP address, not the one for her network. It looked as if her DNS had been hijacked.
I did an IP renew, then rebooted, and all was working again. She has no idea what she was doing that caused it, and there wasn't anything obvious about. It's been OK since (well, she hasn't called again).