March 31, 2009— -- One false click and a stranger can take over your computer.
Experts estimate 10 to 15 percent of personal computers in the U.S. have been taken over and harnessed together into powerful illegal computing tools called "botnets." Some experts believe these botnets are so massive that the criminal hackers who run them have more computing power than the U.S. government.
"It's a very dangerous tool," said Chris O'Ferrell, a computer security consultant and an "ethical hacker."
O'Ferrell and two other computer security experts showed ABC News how criminals can easily gain access to your computer. You open an unknown e-mail attachment, which infects your computer with a virus. The virus hijacks your computer, harnesses its processing power for its own purposes, and then links your computer with hundreds of others to essentially create a supercomputer -- the botnet.
With a click, hackers can see everything you do online -- including your passwords and bank balances. Theoretically, they can even watch movements you make around your own home, if your computer has a Webcam attached to it. Most owners never know their computer is under someone else's control.
"This is organized crime at a global level that is in control of these botnets," O'Ferrell said. "It is a very profitable and very powerful tool, and they are going to use it as much as they can until somebody stops them."
Criminals use botnets to send out 80 percent of the world's spam, to steal people's financial identities and to crack codes that allow them to make massive data breaches. Criminals also use botnets to send overhwhelming amounts of data to companies' Web sites.
The data overload causes the companies' servers to crash. And then the crooks demand a ransom before allowing the Web site to get back online again. This is called a "denial of service" attack and was even used as a form of warfare during Russia's conflict with Georgia.
"It is a serious threat because the cyber criminals recognize that by harnessing the power of tens of thousands or hundreds of thousands of computers, they have that many more opportunities to get to you," said Shawn Henry, chief of the FBI cyber crime division.