Snow Leopard, the highly anticipated new operating system for the Mac, will be released ahead of schedule Friday, Apple announced Monday. The Mac OS X Snow Leopard will be available as an upgrade to the current Leopard system for $29 and can be pre-ordered now, the company said. “Snow Leopard builds on our most successful operating system ever, and we’re happy to get it to users earlier than expected,” said Bertrand Serlet, Apple’s senior vice president of software engineering