Microsoft has announced is offering the public a free trial of its upcoming Windows 8 operating system from today.

Windows 8 will run on tablets as well as desktops and laptops – and is a sea-change for Windows that is seen as a make-or-break opportunity for Microsoft.

Tablets and cloud computing have made Bill Gates’ vision of “a computer on every desk and in every home” seem quaint – Windows 8 aims to adapt the iconic operating system for life in the mobile world of tablets.

Windows 8 will come in two variations, one that works on desktops and laptops, and a new version for the ARM microprocessors in tablets, smartphones and other portable devices.

There is no set release date, but it’s widely expected to be available in autumn – offering Microsoft Office free on tablets, which could be a “killer app” that puts Windows 8 machines ahead of Apple devices.

In both versions, Windows 8 features a completely new interface, borrowed from what Microsoft calls the “Metro” style of the current Windows Phone software.

Download Windows 8 free trial version from here.

Early reactions have been highly positive.

Windows 8 will come in two variations, one that works on desktops and laptops, and a new version for the ARM microprocessors in tablets, smartphones and other portable devices

Microsoft is also opening an Internet “Windows Store” where users can download applications for the operating system.

Applications are free for those testing out the beta version, but would include both free and paid versions after the operating system is released.

The test version was downloaded by people from more than 70 countries as Microsoft gave its presentation about Windows 8, but the company didn’t immediately disclose the number of downloads.

Windows 8 Features

Touchscreen compatible – the new OS will work with tablets and portable devices. There will also be a version for PC’s.

Fewer windows – instead of “stacks” of windows, as in previous versions, the OS is built around clean ’tiles’ that deliver information to the home screen.

Free versions of Word, Excel and Powerpoint – Windows 8 will offer Office free to tablet users.

Business-friendly tablets – Windows 8 will work smoothly with Microsoft Exchange email systems, so could be highly popular with business.

Apps shared with phones – apps in Windows 8 will be built to work smoothly with phones (such as Nokia’s) running Microsoft’s Windows Phone OS.

Easier passwords – users will be able to unlock PCs with a PIN, or by circling details in a picture.

No Start button – instead, users will access a menu from a “hot corner” in the bottom left of the screen.