Check for proxy settings in the browsers. Particular IE because its proxy settings can affect other things.
Download a couple malware scanners like Malware Bytes and AVG Free and run a quick scan. Then a full scan overnight if nothing is found. Some malware will cause connection problems.
Check your hosts file for bad entries. Malware will sometimes put entries there that would cause redirect your browser requests to some other address.
Also try to run a command like
ping google.com. If it gets a response then it's probably not a host file issue, but could still be a browser setting issue like a bad proxy.