As many as 64,000 Americans could be knocked offline Monday as a result of malware unless they change their DNS server addresses, according to a report.
The problem is exclusive to Windows users, but you can check if your computer is infected by clicking on this link. If you are, call your Internet provider, let its representatives know you were affected by the DNS Changer Malware and have them give you a new DNS address, which should solve the problem.
However, that link was down at least for some time Thursday morning, likely due to high amounts of traffic, so if that's the case, there are also various links on this help page to assist you in checking for the infection manually.
Google and Facebook have also been Good Samaritans and have been issuing alerts if their sites suspect infected users are logging on.