this is what happend to me so i hope it's usefull for others.
my router was infected with a dns changer trojan. it kept certain sites and anything from down loading. several different anti programs including avg 8.0 never found it. bleeping computer .com helped me with malwarbytes anti malware, good program
i ran in cmd prompt ipconfig\all and found my dns server was set to 85..... instead of 192.... so i reset it manualy by going control panel, network and sharring center, manage network connections, right click on your connection, selct properties, internet protocal version 4 and 6,select properties, make sure it's not set to an 85 number but to obtain automaticaly.
if you see an 85.... number in the dns server get malwarebytes install and run. then cmd prompt type ipconfig\flushdns, if it says it needs a higher elevation run this in safe mode. make sure to reset your router as well.
last but not least if you still can't update try right clicking on your connection again but instead of properties click diagnose to make sure it's still not trying to communicate with the wrong server. hope that was atleast helpfull to somebody.