LOS ANGELES, Sept. 1 (Xinhua) -- Microsoft Corp., the world's largest software maker, made available its test version of Windows Vista operating system to customers on Friday.

This is part of Microsoft's efforts to deliver the program to consumers by next January.

About 15,000 Microsoft customers and partners gained access to the test version, called release candidate 1, Kevin Kutz, a director in Microsoft's Windows group, said.

More than 500,000 others will get access on Sept. 5, according to a report on on Microsoft's TechBeta website.

Already more than two years behind schedule, Microsoft plans torelease Vista to corporate customers in November before the software becomes generally available in January.

Whether Microsoft makes that deadline will depend on feedback from the current round of testing, market analysts said.

"Microsoft is committed to making its deadlines, but the decision is now out of Microsoft's hands," Joe Wilcox, a Kensington, Maryland-based analyst at JupiterResearch, said. "It'sup to the testers and what they find.''

A further delay may cost the company hundreds of millions of dollars in revenue, Chief Financial Officer Chris Liddell said in July.

The holdup is already expected to shave 4 billion dollars in revenue from PC industry sales this year, according to market research firm Gartner Inc.

Microsoft, based in Redmond, Washington, is working to finish on time and may offer discounts on upgrades so customers won't postpone holiday PC purchases because the machines don't contain Vista.

Amazon.com Inc. is already taking orders for Vista on its website. The online retailer lists the price for Vista Ultimate, the most expensive retail product, at 399 dollars for a full version and 259 dollars for an upgrade. An upgrade to the most basic home version costs 100 dollars.

Altough the operating system is in great shape, experts expected that a lot of testing would have to be done depending on customers' feedback.

Shares of Microsoft rose 14 cents to 25.84 dollars at 4 p.m. New York time in Nasdaq Stock Market composite trading. They have declined 1.2 percent this year. Enditem