The "Live Anywhere" service will be available as part of Windows Vista, Microsoft's next-generation PC operating system. The consumer version is scheduled for release early next year.
"It means that you have one online community," Bill Gates said in a news conference. "This platform can really unleash developers to do amazing new things."
The system would extend the company's existing Xbox Live service for the Xbox 360 console into millions of Internet-enabled PCs and cellular phones. No pricing information on the new service was announced.
"The vision here is each platform plays its own role. We really think we're in a unique position," Gates said. "Microsoft is probably the only company that can pull this off."
In a demonstration, an Xbox 360 gamer using Live Anywhere was able to send an invitation to a friend on a PC. After the PC user agreed, the two connected for a cross-platform online gun battle.
In another example, a race car's stats were modified on a cellular phone, its appearance was edited in a paint program on the PC and then it was raced on a track on the Xbox 360.