Microsoft has announced a move to release virtualisation software for free, which will allow computers running on rival products like Apple's OS X or Linux to simultaneously run Windows. The company will relinquish all license claims on its Virtual Hard Disk Image Format, in a move which the company claims is just as much about working with their rivals as with their partners.

Currently, the virtualisation market is dominated by VMware Inc., who has some 95 to 98% market share. Virtualisation is set to grow in popularity, with chipmakers such as Intel and AMD releasing processors that have extra instructions to help virtualisation performance. This has many advantages for users, such as backup and recovery, security, image management and patching, disk conversion and lifecycle management and provisioning. It’s a tempting market for Microsoft to try to penetrate and gain dominance in, and they believe that the only way to accomplish this is to try to promote their own standard.

The existing Virtual Hard Disk technology has been available since May 2005, and captures the entire virtual machine operating system and the application stack in a single file. The technology is currently utilised in Microsoft's Virtual PC 2004 and Virtual Server 2005 products, and is due to make its way into the Windows Longhorn server product. The company has pledged to make its patented virtualisation technology available to developers in September of this year to help promotion, and has made the VHD format available for download with or without registration. Users who register have the benefit of being informed of product updates.

"By having the VHD specification available under [an open source license we have dubbed the Open Specification Promise (OSP)], the technology is viable for any development or business model," Bob Muglia, senior vice president of the Servers and Tools business at Microsoft, said in a statement. "We continue to increase our commitment to interoperability in our products."