Some people claim that DrCom would cause such a problem, and some guys believe that the OS has somehow been modified... Anyway, Avast! is blocking port 80, which is vital for browsers.
Let's try everything we can think of(anyway,things won't get worse...):
In 'troubleshooting', add 'localhost' to 'ignore address' and check if it works;
Change the proxy of your browser to whatever you like and modify the corresponding port in 'HTTP port(s)' of troubleshooting.
Lastly, if nothing works, uninstall Web Shield straight away...
You can disable it temporarily and surf some websites that are confirmed to carry malwares, usually, Network Shield would block the elements or the whole webpage, so that you can uninstall Web Shield decisively...