Your computer may have been infected by hackers who figured out a way to redirect your Internet searches to bogus websites for profit.
"We call it DNS changer and it does just what the name implies -- it changes DNS settings. So it's gone in and reconfigured it so that when you go to browse somewhere, it can take you anywhere it wants," said Kevin Healey, director of Norton Security Response.
Security experts at Symantec, makers of Norton Anti-virus software, say infected computers probably got the DNS changer through email attachments or clicking on infected links.
"You probably don't even know that you were infected," said Healy. "Now the good news is the FBI has captured and shut down the bad guys behind all this, but the bad news is that your configuration settings may not have been changed back to the right thing."
That means if your computer is infected and you don't take steps to protect yourself, you could lose all access to the Internet. The problem affects both PC and Mac users.
The FBI says 4 million computers in 100 countries were infected. The agency said it has shut down the servers used by the hackers.
"It's really terrible, because I have an Internet business and the last thing I want is that shut down," said Tim Becherer of Santa Monica.
Experts say it's easy to check your computer to make sure it hasn't been infected.
"You can go to www.dns-ok.us. You'll have a big green screen come up if you're good. If you're bad there will be a red screen and it will tell you what to do to fix the problem," said Healy.
Alternatively, anyone wishing to test their computers for the virus can visit http://www.mcafee.com/dnscheck.
Some Internet service providers say they have a system in place for anyone affected.