Microsoft on Monday changed its mind for the third time by allowing Vista Home Basic and Vista Home Premium to be used as guest operating systems on a virtual machine.

Mac users and others wanting to run Vista virtually have until now had to buy the most expensive Business and Ultimate versions.

Microsoft had briefed reporters in June that it was going to expand Vista's virtualisation options, but then for reasons that were never made clear, it never announced such a move.

Microsoft's argument against allowing Home versions of Vista from being virtualised was that it represented some security risks. Microsoft said it was not the case that the Ultimate or Business versions had less of a security risk than the Home editions, but rather that by limiting virtualisation to the pricier versions, ideally only more technically sophisticated people would make the move.

Monday's move is likely to be welcomed by enthusiasts, Mac users, and virtualisation software vendors such as Parallels.

Microsoft group product manager Patrick O'Rourke said in a telephone interview on Monday: "Now is the right time, we believe, to make it easier for technical enthusiasts...to experience and see if virtualisation is right for them."

O'Rourke wouldn't go into the specific thinking behind the delay. "There was some internal discussion still occurring at the time," he said.

Microsoft on Monday also said it is buying Calista Technologies, a California-based company that has technology to improve the performance of remote desktops. The company has a patented approach for creating a virtualised graphics processor, which can help with 3D and other image-rendering tasks when doing so-called "presentation virtualisation." Microsoft also expanded its alliance with thin-client specialist Citrix Systems.

Savings were also announced for large businesses that want to run Vista on a server and use either a PC or thin client to act as a terminal to display the information.

O'Rourke said that businesses that are part of its Software Assurance program will be able to use the technology by paying a licence fee. For Windows PCs, the charge is US$23 per machine a year, while thin clients will require a fee of about US$110 annually.

That represents a price cut of about 25 percent to 50 percent compared with Microsoft's original plans.