If this has just started happening & you had no issues before & you haven't changed isp or modem then i would look at virus or some other infection,you could try cleaning the cache with ccleaner,then try your proxy settings,web search for solutions then if no go start a topic in forum,oh yeah make sure antivirus software or spyware scanner isnt blocking it

This seems to be an issue mostly related to Norton blocking things. Check your firewall settings, too. Mos def NOT a router issue.

try going to command prompt and typing these one line at a time, press enter after each line. scorpnz may be right about a virus, tho.
netsh winsock reset
netsh interface ipv4 reset
netsh interface ipv6 reset
ipconfig /flushdns