LOS ANGELES — Hell froze over yesterday. Pigs flew. And Apple Computer, for the first time, unveiled software allowing Microsoft's Windows operating system to run on Apple's PCs.

That's a huge change for the iconoclastic PC maker, which has long shunned Windows even though it runs on more than 90 percent of PCs. Now Apple is moving toward the mainstream and easing its decades of rivalry with Microsoft.

Apple has been on a three-year roll with the smash success of its iPod digital music player. But it remains a minor player in PCs, with just 4 percent of the U.S. market. Microsoft allies such as Dell and Hewlett-Packard dominate.

So it introduced free software that lets users of some new Apple Macintosh PCs run a version of Windows called XP. But it won't be easy or cheap. Most customers will have to fork over about $200 to buy a copy of Windows. And Apple won't help with installation.

Still, says analyst Tim Bajarin at researcher Creative Strategies, the move is "very significant" for a company that has long marched to its own beat. Investors seemed to agree. Apple shares soared nearly 10 percent yesterday, closing at $67.21.

The new software, Boot Camp, makes Macs "even more appealing to Windows users considering making the switch," says Apple senior vice president Phil Schiller. But he emphasized that "Apple has no desire or plan to sell or support Windows."

Microsoft said in a statement, "Windows is a great operating system. We're pleased that Apple customers are excited about running it, and that Apple is responding to meet the demand."

Apple has been telling consumers for years that anything they wanted to do on a Windows machine could be done with a Mac. It pointed to popular programs such as Microsoft's Office and Intuit's Quicken, available in both Windows and Mac formats.

But some other programs, such as some video games, are Windows-only. And many Web sites offer video with copy protection from Microsoft, preventing them from being viewed on Macs. They include MTV.com, ComedyCentral.com and Yahoo's music video offerings.

Apple is aiming for more customers like Jeff Prus, 35, a Windows user in New York City. Prus has long wanted a Mac. But he has stuck with Windows because his employer, American Express, uses it.

"I will definitely consider buying a Mac now that I can have the best of both worlds," says Prus. "I'm thrilled."

Some users already were making the switch yesterday. Oliver Breidenbach, a software developer in Munich, Germany, installed Windows on an iMac. He said it took him about an hour and 20 minutes to complete the installation.

Getting Windows onto the new Macs was something he said he expected to see from a third-party developer, not Apple. "I just never thought Apple would do this," he said.

Yesterday's release doesn't end the 25-year rivalry between Apple and Microsoft.

Apple is not selling PCs with Windows already installed, as Microsoft partners Dell and Hewlett-Packard do. Instead, users must buy a Mac, then download Boot Camp from Apple's Web site. Then they must buy a copy of Windows XP and install it themselves. When they do so, they'll have two operating systems, Windows and Apple's Tiger.

But users can't simply switch back and forth between the two. Instead, they need to reboot the computer to use one or the other.

And while Windows programs will work on the Mac, the same can't be said for Mac programs in XP. Apple software such as photo-management program iPhoto and movie-editing tool iMovie will continue to be Apple-only.

Apple says it won't sell XP in its retail stores. And the tech support specialists in those stores called "Mac Geniuses" won't offer help.

For those reasons, "Millions of people aren't going to run out and buy Macs," says PC analyst Samir Bhavnani at researcher Current Analysis. Still, he says, "It could be an important first step toward making Apple more than just a niche player in the PC market."