The long-awaited rollout on to Microsoft Windows is key to the success of the Apple service, if it is to hit its goal of reaching the 90% of PC owners who use Windows.
Steve Jobs, the Apple founder and chief executive, said: "This has been the birth of legal downloading."
The new iTunes service promises a wider library of songs and new features in an attempt to make it appeal to a wider audience. The iTunes software is now available for free download for Windows, as well as Macintosh, users at the site.
The new service will give users access to 400,000 tracks by the end of the month, double the number that were available when it launched in April.
The launch of the new service saw Jobs hook up in a live link with U2's Bono, Mick Jagger and Dr Dre, before a live performance by singer-songwriter Sarah McLachlan.
"It's like the Pope of software meeting up with the Dalai Lama of integration," Bono said.
The company has sold 13m songs in the five-and-a-half month period since launch. It aims to have sold 100m by the time the service is a year old and is boosting its effort with the two-month Pepsi giveaway promotion, which launches in February.
Apple also unveiled a series of add-ons for its digital music player, the iPod, to allow it to be used for voice recording and digital photo storage, with digital photos automatically downloaded into Apple's iPhoto software when users place the iPod into its cradle, which is plugged in to a Macintosh computer.
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