If someone you know is having problems connecting to the Internet starting on about July 9, 2012 then they might be affected by a DNS changer malware which the FBI has been investigating.
The FBI has a page of resources including a way to check your own computer.
In addition, it is very important to run an up-to-date virus checking program on the affected computer. Infection by one type of malware usually means that a computer will be affected by others as well.
Microsoft's Malicious Software Removal Tool is a good resource as is the Microsoft Safety Scanner.
On Mac, the App Store contains a free version of Symantec/Norton called iAntivirus.
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