Microsoft said on Tuesday it would allow anyone to use its specifications for "virtual" drives, which enable one computer to run several operating systems, with the promise never to sue for infringement of its legal rights. The Microsoft virtualisation software has been available for more than two years, but as computers become more powerful the use of virtualisation is expected to mushroom, the company said at a news conference.

The software permits the easy use of several operating systems on one machine. So, for example, dangerous software could be installed on a virtual machine without affecting the host computer.

The company's specifications will be made available to anyone who wants to use them under an "Open Specification Promise," introduced last month.

The company said the license was "an irrevocable promise from Microsoft to every individual and organization in the world to make use of this patented technology for free, now and forever when implementing specified open standards."

Earlier this year, Microsoft said it would team up with Linux supplier XenSource to supply the virtual specifications to permit Windows and Linux to run on the same machines.

Such virtual software is also made by VMWare, a unit of EMC Corp., which helped pioneer the market.

The question of making software open to others has been at issue in Brussels, where the European Commission has ordered Microsoft to share protocols with rival makers of servers.