Some of this year's hottest gifts, especially for teens, are those tiny, portable digital music players such as the pack-leading Apple iPod (estimated holiday sales - 4 million) and competitors like Creative Zen Micro and Rio. About the size of cell phones, MP3 players can carry music anywhere and store thousands of songs copied from CDs or downloaded from the Internet.

But technology security experts warn that many of this holiday season's millions of newbie MP3 player owners don't know what dangers lurk behind some music.

"The risk has skyrocketed," says Kraig Lane, group product manager at the computer-security products manufacturer Symantec. "The bad guys are putting evil agents into music files and even videos that we are downloading. Music files especially. And you don't know it's there."

The big problem is that some music services - particularly the free and legally questionable peer-to-peer (P2P), file-swapping networks like Kazaa, BearShare and LimeWire that connect millions of home-computer users - deliver something in addition to free software and music. They sneak in adware - or, even worse, viruses and spyware.

Even reputable online music stores sometimes install adware. Considered the most benign of such programs, adware hides in the background of a computer to track user online behavior and report it to advertising companies so they can target ads. It's legal, and users often grant tacit permission to receive it when accepting licensing agreements at Web sites.

Such downloading has become big business for those sites. Apple announced last week that music fans had downloaded more than 200 million songs from its iTunes Music Store since its launch in 2003. Featuring more than 1 million tracks at 99 cents each, iTunes now sells nearly 5 million songs a week. The online service eMusic said recently that it is selling 1.5 million downloads a month. And there are more than a dozen other online sites either selling or sharing, free of charge, millions of downloaded music files. Even Wal-Mart now sells music online to download.

More sinister and malicious than adware is spyware, which scours the hard drive for personal and financial data, such as credit card numbers, and reports it to crooks.

"Kids are attracted to the file-sharing Web sites where they share little music snippets with each other - but those snippets can now carry these evil agents imbedded in them," says Lane, adding that the problem has bumped up in scale and severity over the past year. "It's like a Trojan horse."

What to do about spyware?

Not downloading files from disreputable or questionable sites is a start. Then there are several free online programs that scan your hard drive and eliminate spyware, such as Spybot and Ad-Aware. At its Web site, Symantec offers a free service that searches a computer for spyware.

And, says Lane, it's essential in these dangerous times online to protect home computers with an up-to-date firewall, antivirus and anti-spam program, such as Symantec's Norton Internet Security.

Other top-rated programs include McAfee's Personal Firewall and VirusScan, and Trend Micro's PC-cillin Internet Security.