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.

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.

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.

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 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.

Security experts say the malicious software is apparently being loaded at the final stage of production, when gadgets are pulled from the assembly line and plugged in to a computer to make sure everything works. If the testing computer is infected -- say, by a worker who used it to charge his own infected iPod -- the digital germ can spread to anything else that gets plugged in.

Security experts say the infections point out an avenue of attack that could be exploited by hackers.

"We'll probably see a steady increase over time," said Zulfikar Ramzan, a computer security researcher at Symantec Corp. "The hackers are still in a bit of a testing period -- they're trying to figure out if it's really worth it."

Thousands of people whose antivirus software isn't up to date may have been infected by new products without even knowing it, experts warn. And even protective software may not be enough. In one case, digital frames sold at Sam's Club contained a previously unknown bug that not only steals online gaming passwords but disables antivirus software, according to security researchers at Computer Associates.

Consumers can protect themselves by running an antivirus program and keeping it up to date.

The companies whose products were infected in cases reviewed by the AP refused to reveal details about the incidents. Of those that confirmed factory infections, all said they had corrected the problems and taken steps to prevent recurrences.

-- The Associated Press