Hundreds of thousands of people around the world could be in for an unwelcome surprise Monday, unless they check their computers now for malware that infected their machines a year ago.
The virus caused by Eastern European computer hackers will make it impossible to access the Internet.
Facebook and Google, Web sites used by billions of people every day, are alerting users who they suspect may have infected computers. They also provide information on what people can do to fix the problem.
The hackers used an online advertising scam to take control of nearly 600,000 computers last year, harvesting information about users and directing them to fraudulent Web sites.
The U.S. Federal Bureau of Investigation smashed the computer ring and temporarily set up two Internet servers so that users can remain on line.
But that temporary fix will be closed down Monday.
Along with Facebook, Google and some global computer security firms, the FBI can provide information on how to find out if a computer is infected and how it can be fixed.