This might be off the wall, but I have to ask: do you have a hosts file?
Could somebody have edited it and be pulling a joke on you?
Check out:
http://www.accs-net.com/hosts/what_is_hosts.html"if you put ad server names into your Hosts file with your own computer's IP address, your computer will never be able to contact the ad server. It will try to, but it will be simply calling itself and get a "busy signal" of sorts. Your computer will then give up calling the ad server and no ads will be loaded, nor will any tracking take place. Your choices for blocking sites are not just limited to blocking ad servers. You may block sites that serve advertisements, sites that serve objectionable content, or any other site that you choose to block. "
Just for fun, use the search function to search all your hard drives for
files containing hosts in the filename.
Look inside any found with notepad. Find anything suspicious?