Browser working normally on desktop PC all morning, then started giving nothing but Page Not Found error. Same problem on both Firefox and Explorer. Wouldn't even let me access my router using the numeric IP address.
Internet still accessible, was able to start up online session of internet game (World of Warcraft).
Browsers working normally on laptop in same room. Both machines running Windows 8.
Shared files on the two PCs no longer accessible through network.
NIS would not start up at all so couldn't do any diagnosing through it or turn it off.
Finally had to uninstall NIS then reload it. This seemed to have been the culprit as everything now seems to be back to normal.