Instead, Microsoft's Kevin Johnson said Vista would be shipped "when it is available".

He was speaking at the firm's annual financial analyst meeting.

Hesitation?
Microsoft originally intended to release Vista - the first major update since Windows XP was introduced five years ago - in the second half of 2006.

Yet in March of this year it postponed the release until 2007, and last month Microsoft chairman Bill Gates predicted there was only an "80% chance" it would be shipped on time.

Mr Johnson, Microsoft's co-president of Microsoft's platforms and services unit, said: "We are going to ship the product when it is ready, and we are just going to take it milestone by milestone."

Goldman Sachs analyst Rick Sherlund said Mr Johnson appeared to be hesitating over the release date.

Vista will be available for corporate customers in November.

Mr Johnson also told the analyst meeting that Microsoft was on target to achieve revenues of between $14.3bn (£7.7bn) and $14.5bn (£7.8bn) in its current fiscal year.

Those installing Vista will be encouraged to send in feedback about their experiences with the pre-release program.

The final version of Vista for home users is due to be released in January 2007.

Testing teams

Vista is the long-awaited update to Microsoft's widely used Windows operating system.

It is the biggest revamp of the software for five years. It makes numerous changes to the inner workings of Windows to improve, among other things, the way it handles networking and audio.

Those registering for the test version will get two versions of the software - beta 2 and release candidate 1. The beta is available now and the other copy will be released later in 2006.

Microsoft is limiting the numbers of registrations but has not said when it will stop taking applications.

Before now Microsoft has only let small numbers of Windows developers play around with early editions.

The trial software is also aimed at developers but also includes other technology professionals as well as home users happy with installing software and troubleshooting problems on their PC.

Risky business

A warning on the page announcing the Vista preview says the software should not be run on a production machine or the main computer in someone's home. It adds that, if anything goes wrong during installation, users will not be able to roll back to the former set up on their PC.

"Some risks of using beta operating systems include hardware and software incompatibility and system instability," reads the warning.

Only those happy to tinker with a PC should install the software
Those keen to get hold of the software and try it out should consult the checklist which will tell them how powerful a PC needs to be to run the software. On under-powered PCs, some Vista features, such as the 3D interface, will not work.

The edition of Vista being made available is the Ultimate version which combines all the elements of the software intended for both home and business users.

Versions are being made available in three languages - English, German and Japanese.

The software can either be ordered on a DVD or downloaded from the Microsoft site. Getting the DVD involves a small payment for shipping fees.

The 32-bit version of the software weighs in at 3.5GB and the 64-bit edition 4.4GB and Microsoft estimated it would take about 21 hours to download the larger package over a broadband link running at 768kbps.

Once downloaded Microsoft recommends that users burn the data onto a DVD.

Microsoft also warned that users who get hold of this early version of the software may not be able to upgrade it to the version that makes it into the shops.

Instead they may have to buy the new software and install it all over again.