I had the same problem last week. My desktop web browsers could'n access google, youtube and other sites. I thought it's a ISP problem, but while some sites were working flawlessly others didn't at all, for instance performing a download at max. speed (10-15 Mbps) worked well, but i couldn't access the main page from google which has a very simple user interface. At the same time the internet worked well on all sites on my smartphone.
It took me some (unnecessaryly waisted) time to try to find out what the problem is spending hours on searching on the internet and then something crossed my mind. I just updated by accident my avast free to 10.3.2223 version and the problem dissapeared.
I don't know anymore what my previous software and virus definition were.