A virus has been floating around for a couple years that changes your dns. dns is used to direct you to the correct web page when you type the domain into your browser. (microsoft.com)
This virus uses a different dns server to give your computer the wrong ip address to a web site. The viral dns server was shut down last year and replaced with a proper temporary server.
That temporary server will go offline in July leaving infected computers unable to access the web.
A web link has been set up to see if you are infected. Use this link to test your computers dns: http://dns-ok.us/
You can read more about this virus at: http://www.msnbc.msn.com/id/47537422/ns/technology_and_science-security/t/google-notify-users-infected-dnschanger-trojan/#.T9Ee0tVYvPY