Did you include network in your pro file?
CONFIG += core network
Use an app like Depency Walker to check which dll will be loaded. Eventuall you have an older QtNetwork.dll installed in your system (evtl. $WIN$/system32/) which will be called at higher priority than your QtNetwork.dll in your QT-Directory/Development-Directory.
- Also check your path-variable and qt-environment variables.
If you find a wrong dll try to rename it and restart. Another option would be to do a full clean and qmake, but most probable reason will be a wrong .dll being loaded.