|Are other machines on this network able to visit these sites? If not the network is configured to block them, perhaps through DNS filtering or at the router.|
If other machines can get to the sites, open a command window Winkey + R, type cmd, Enter, and in the window type 'ping <address of a site you are having problems with>'. If you get a ping reply, your machine is ok and the problem is with the browsers (unlikely).
If there is no ping, search the Windows folder for "hosts" files, and check for any rerouting (though you said there was no malware, so this too is unlikely).