Observers are calling the launch of Apple Computer's digital music service the iTunes Music Store an overwhelming success, Billboard Bulletin reports. The service, which went live Monday, sold an estimated 275,000 tracks at 99 cents apiece in its first 18 hours, according to major-label sources.

All songs -- including exclusive material and videos from U2, Eminem, Sheryl Crow, Bob Dylan, Sting, and others -- can be previewed for free as 30-second samples. All can be burned to a CD and are automatically synched to an iPod.

The feat is especially remarkable when considering that the offering is available only to the limited universe of users of Apple computers. The launch thereby sets the stage for a race between a host of media and technology companies to create and effectively promote similar services for the much bigger Microsoft-equipped PC market.

"There is going to be a race to see who can get to the Windows market and start to replicate this," says the head of new media at one major label. "The question is [whether] someone else wants to put up the kind of money that Apple is to let people know they're there."

Apple says it plans to make iTunes compatible with the PC by the end of the year. Sources tell Bulletin that two major labels have already cut wholesale agreements with Apple for the Windows version of the service.