Of all the sinister things that internet viruses do, this might be the worst: They can make you an unsuspecting collector of child pornography.

Heinous pictures and videos can be deposited on computers by viruses — the malicious programs better known for swiping your credit card numbers. In this twist, it's your reputation that's stolen.

Michael and Robin Fiola in their Rhode Island home with the forensics report, at left, that exonerated Michael. (Matthew Healey/Boston Herald/Associated Press) Pedophiles can exploit virus-infected PCs to remotely store and view their stash without fear they'll get caught. Pranksters or someone trying to frame you can tap viruses to make it appear that you surf illegal websites.

Whatever the motivation, you get child porn on your computer — and might not realize it until police knock at your door.

An Associated Press investigation found cases in which innocent people were branded as pedophiles after their co-workers or loved ones stumbled upon child porn placed on a PC through a virus. It can cost victims hundreds of thousands of dollars to prove their innocence.

Their situations are complicated by the fact that actual pedophiles often blame viruses — a defence rightfully viewed with skepticism by law enforcement.

"It's an example of the old 'dog ate my homework' excuse," said Phil Malone, director of the Cyberlaw Clinic at Harvard's Berkman Center for Internet & Society. "The problem is, sometimes the dog does eat your homework."

The AP's investigation included interviewing people who had been found with child porn on their computers. The AP reviewed court records and spoke to prosecutors, police and computer examiners.

One case involved Michael Fiola, a former investigator with the Massachusetts agency that oversees workers' compensation.

In 2007, Fiola's bosses became suspicious after the internet bill for his state-issued laptop showed that he used 4½ times more data than his colleagues. A technician found child porn in the PC folder that stores images viewed online.

Fiola was fired and charged with possession of child pornography, which carries up to five years in prison. He endured death threats, his car tires were slashed and friends shunned him.

Fiola and his wife fought the case, spending $250,000 on legal fees. They liquidated their savings, took a second mortgage and sold their car.

An inspection for his defence revealed the laptop was severely infected. It was programmed to visit as many as 40 child porn sites per minute — an inhuman feat. While Fiola and his wife were out to dinner one night, someone logged on to the computer and porn flowed in for an hour and a half.

Prosecutors performed another test and confirmed the defence findings. The charge was dropped — 11 months after it was filed.

The Fiolas said they have health problems from the stress of the case. They said they've talked to dozens of lawyers but can't find one to help them sue the state, because of a cap on the amount they can recover. "It ruined my life, my wife's life and my family's life," Michael Fiola says.

The Massachusetts attorney general's office, which charged Fiola, declined interview requests.

At any moment, about 20 million of the estimated one billion internet-connected PCs worldwide are infected with viruses that could give hackers full control, according to security software maker F-Secure Corp. Computers often get infected when people open email attachments from unknown sources or visit a malicious webpage.

Pedophiles can tap viruses in several ways. The simplest is to force someone else's computer to surf child porn sites, collecting images along the way. Or a computer can be made into a warehouse for pictures and videos that can be viewed remotely when the PC is online.

"They're kind of like locusts that descend on a cornfield: They eat up everything in sight and they move on to the next cornfield," said Eric Goldman, academic director of the High Tech Law Institute at Santa Clara University. Goldman has represented web companies that discovered child pornographers were abusing their legitimate services.

But pedophiles need not be involved: Child porn can land on a computer in a sick prank or an attempt to frame the PC's owner.

In the first publicly known cases of individuals being victimized, two men in the United Kingdom were cleared in 2003 after viruses were shown to have been responsible for the child porn on their PCs.

In one case, an infected email or popup ad poisoned a defence contractor's PC and downloaded the offensive pictures.

In the other, a virus changed the home page on a man's web browser to display child porn, a discovery made by his seven-year-old daughter. The man spent more than a week in jail and three months in a halfway house, and lost custody of his daughter.