For years it has sold downloadable versions of other, far smaller applications. But this is a new distribution method for Windows and Office, which in the past Microsoft has sold only on discs or preloaded on new PCs.

For its download program, company is relying on digital locker Web technology, which stores the alphanumeric license keys that gives users the rights to download the software and also enables them to resume downloads that have been interrupted.

A Microsoft spokesperson said electronic software distribution, or ESD, was the future of software distribution. She pointed to a recent Gartner report that forecasted that by 2008, 80% of all consumer software and 50% of all enterprise software would be delivered via ESD. IDC reckons the ESD market will see a compound annual growth rate of 34% through 2009, she added.

This doesn’t mean Microsoft is moving toward all ESD and away from retail, said the spokesperson. This is just another promising channel for distribution.

Indeed, Redmond expects most of its consumer customers will get Vista when they buy a new PC. And it isn’t extending its new Windows Marketplace downloadable option to enterprise customers with volume licenses.

Microsoft’s spokesperson declined to answer when or if enterprises will be able to download Vista online at some future. Microsoft has just introduced these programs and has no news to share about future plans, she said.

The download program will be available in North America only. It is targeted at Windows XP users who want to upgrade to Vista without buying a new PC. However, Vista has new hardware requirements over XP and may require a hardware upgrade in any event.

When Vista for consumers is launched on January 30, Microsoft will offer four downloadable versions: Home Basic, for $199; Home Premium, for $239; Business, aimed at SMBs, for $299; and Ultimate, for $399.

The company also plans to offer two additional methods to upgrade to Vista. One is Windows Anytime Upgrade program, in which Vista users can upgrade to a more expensive version by clicking on the Start menu to be directed to a Microsoft Web site where they can download software keys to enable new Vista features already on their hard drive.

Alternatively, customers who have already bought the most expensive Vista version, Ultimate, can buy two copies of Vista Home Premium for just $50 each for use on other machines.