I´ve been struggling with a rather annoying problem lately. I´ve got a single client unable to reach SharePoint UNC-paths. After some troubleshooting i´ve found it related to the WebClient-service being disabled on the computer, however simply starting the service gives an error window telling me the service was started and then stopped again. No logs, no warnings in event log, nothing!
Finally i´ve found a Swedish post mentioning this problem being connected to the hardware, an intel wireless card interferring with the service. I inactivated the computer’s (Lenovo ThinkPad) wireless card and ta-da, the WebClient service would start.
This seems to be one solution however there are a couple other things that might be your problem, ie i´ve found a rather extensive amount of posts describing that the C:WindowsSystem32Driverstcpip.sys being corrupted. Simply replacing it with a fresh copy might work for you.