The Microsoft Live network already has more than 6 million users all buyers of an Xbox console. the PC move could well double this base very quickly.

The network is a venue for online game play and includes gamer identity, lists of online friends to play with, in-game chatting and text messaging, a marketplace and other features.

"The PC is the largest gaming platform in the world," said Aaron Greenberg, Microsoft's group product manager for Xbox Live and the new service, Games for Windows Live.

"So while we feel great about the 10 million Xbox 360s that we've sold, we realize that the market opportunity on the PC is significantly larger than the console has ever been historically."

The Live service will also allow gamers to go head-to-head on their platform of choice, potentially settling the debate over the superior gaming user interface: the mouse and keyboard controls of PC gaming or the joysticks and buttons of an Xbox controller.

Microsoft plans to have three titles available for the PC online gaming platform in 2007, and they're all designed to run on its new Windows Vista operating system.

The blockbuster "Halo 2" title will launch the new service when it becomes available May 8. Later in the year, "Shadowrun" a first-person shooter, and "Uno," the card game, will be available on the platform. "Halo 2" and "Shadowrun" are products of Microsoft Game Studios.

Microsoft sees this as a potential spur to Windows Vista sales. Multimedia improvements in the operating system, such as the Direct X 10 graphics engine, will entice video-game enthusiasts to Vista, Greenberg said.

Analysts see several other business reasons for extending the Live network to PCs."It's an incredible investment just to provide an online gaming channel," and Microsoft is leveraging that investment by extending the service to PCs, said Billy Pidgeon, a video-game analyst at IDC.

There are some 200 million PC gamers worldwide. But only PCs running Windows Vista, the operating system Microsoft launched Jan. 30, would be able to use the Live network at first. IDC estimates that 55 million copies of Windows Vista Home will be sold in 2007.

Still, that represents a vastly larger potential audience for the Live network.

"The network is a distribution channel, a retail channel and an advertising channel," Pidgeon said. "That to me is where the money is."

Greenberg said Microsoft is keeping open the possibility of bringing the service to other versions of Windows, potentially addressing a much larger slice of PC gamers.

When Microsoft Chairman Bill Gates described his company's plans for "Live Anywhere" gaming at the Electronics Entertainment Expo last May, he mentioned gaming on mobile devices in addition to the Xbox and PC.

Greenberg said that's still part of the long-term plan, but he didn't offer any timeline.

"Microsoft is really working to extend Xbox beyond just the living room into this whole ecosystem, which, of course, is a classic Microsoft strategy," said Michael Gartenberg, an analyst with JupiterResearch.

The company has also sought to fortify the Xbox 360's position as a hub of entertainment. In November, it began offering movie and TV downloads through Xbox Live. The console can also record and display live TV when networked to a Windows Media Center PC.

As with Xbox Live, a subscription to the highest level of the Games for Windows Live service will cost $50 a year. Gamers who already subscribe via their Xbox do not have to pay again to use the service on a PC.

A second-tier service that still allows multiplayer gaming is free, but it will not allow PC gamers to play with others using the Xbox 360 console.