I saw two machines in two days with the same symptoms- high speed (dsl) connections, apparently online, modem/routers okay, but zero connectivity. The first one took some time to investigate, as I hadn't seen this specific problem before. But I eventually got the answer after talking to a colleague who's begun seeing the same problem on other machines- the winsock entries in the registry are being corrupted by something. Spyware, virus, or something new, is my guess. My analysis leads to the conclusion that the settings are being changed from accepting a dynamic IP to requiring a static, private IP. It 'gets stuck' on that setting, and will not revert to dynamic. I didn't write down the private IP that had been written in, but it's going to a specific computer's open address. I think computers are being hijacked.
Anyway, between my collegue's clues and Google, I eventually found this site that lays out a solution to the problem. There's a utility available for download that will back up the registry and then reset the registry settings. All you have to do is run the wizard and reboot when it's done, and connectivity is restored.
I urge the techies to add this utility to your toolkits. I have a feeling it may be an important one.
http://home.earthlink.net/~lreynol929/ruXP/Internet/fixing.htmI might add that both machines DID have spyware and trojan problems I had to fix, but I did that AFTER I applied the fix and confirmed it worked properly. So you can see where my suspicions come from.