From iPods to navigation systems, some of today's hottest gadgets are landing on store shelves with some unwanted extras from the factory — pre-installed viruses that steal passwords, open doors for hackers and make computers spew spam.

Computer users have been warned for years about virus threats from downloading Internet porn and opening suspicious e-mail attachments. Now they run the risk of picking up a digital infection just by plugging a new gizmo into their PCs.

Recent cases reviewed by The Associated Press include some of the most widely used tech devices: Apple iPods, digital picture frames sold by Target and Best Buy stores and TomTom navigation gear.

In most cases, Chinese factories — where many companies have turned to keep prices low — are the source.

So far, the virus problem appears to come from lax quality control — perhaps a careless worker plugging an infected music player into a factory computer used for testing — rather than organized sabotage by hackers or the Chinese factories.

It's the digital equivalent of the recent series of tainted products traced to China, including toxic toothpaste, poisonous pet food and toy trains coated in lead paint.

But sloppiness is the simplest explanation, not the only one.

If a virus is introduced at an earlier stage of production by a corrupt employee or a hacker when software is uploaded to the gadget, then the problems could be far more serious and widespread.

Knowing how many devices have been sold, or tracking the viruses with any precision, is impossible because of the secrecy kept by electronics makers and the companies they hire to build their products.

But given the nature of mass manufacturing, the numbers could be huge.

"It's like the old cockroach thing — you flip the lights on in the kitchen and they run away," said Marcus Sachs, a former White House cybersecurity official who now runs the security research group SANS Internet Storm Center. "You think you've got just one cockroach? There's probably thousands more of those little boogers that you can't see."

Jerry Askew, a Los Angeles computer consultant, bought a new Uniek digital picture frame to surprise his 81-year-old mother for her birthday. But when he added family photos, it tried to unload a few surprises of its own.

When he plugged the frame into his Windows PC, his antivirus program alerted him to a threat. The $50 frame, built in China and bought at Target, was infested with four viruses, including one that steals passwords.

"You expect quality control coming out of the manufacturers," said Askew, 42. "You don't expect that sort of thing to be on there."