Originally Posted by deubster

Many possibilities, most of them malware. The things that come to mind first:

1) Check to see if any malware has messed with your HOSTS file.
2) Some malware install a proxy server. Check your browser settings.

See if you can resolve addresses (DNS issue) by going to a command prompt and pinging somebody like yahoo.com - if it displays the IP address, your DNS is OK. After that, run a tool that catches Trojans better than most AV programs, like Malwarebytes.