Earlier this week, the Redmond, Washington software company had indicated that it would reverse its earlier policy, and would allow Vista Home Basic and Home Premium editions to be used under virtualization on the Mac.

This decision appeared to be a move on the part of Microsoft to yield to market pressure, as only the more expensive Business, Ultimate, or Enterprise editions have been approved for virtualization under the company's licensing agreement.

Canceling the Announcement

Virtualization allows several operating systems, or virtual computers, to run on the same hardware . In data centers, virtualization is increasingly viewed as a way to save on hardware, power , and other costs. For Mac users, it is one way to run Mac OS while also running Windows.

The Microsoft announcement was supposed to have been made Wednesday, and a prepared statement sent to news outlets under embargo had touted the company's yielding to user "choice." The decision would have allowed all Vista versions to run legally under virtualization.

Instead, Microsoft decided to keep things as they are, canceling its announcement. "Microsoft has reassessed the Windows virtualization policy," an e-mail sent to news outlets indicated on Tuesday, "and decided that we will maintain the original policy announced last Fall."

Microsoft originally had said the restrictions were due to security issues, and that the ability to counter potential security threats required a higher level of knowledge than is generally available to those who use the home editions. However, when they previewed their first change of heart, Microsoft officials indicated that the company was interested in customer choice and that customers were aware of the security issues.

'Weren't Ready To Go Ahead'

Some observers have noted that security problems, such as rootkits that can operate maliciously and secretly in a virtualized environment, can affect the business editions of Vista as well as the home editions.

"They just weren't ready to go ahead," said IDC analyst Brett Waldman, who added that he thinks Microsoft will eventually change its position again.

He said that, while there were "some security issues," Microsoft is being "overly cautious" by saying that a company is more able to deal with the security issues than a home user is. Waldman added that it might be both concern about security and an attempt to protect the market for the more expensive versions. Other observers have said that another reason for the change-back is because it will help to discourage the use of Windows Vista on the Intel-based Macs.

The move primarily impacts Mac users of the Parallels virtualization software called Parallels Desktop for Mac. Reportedly, Intel CEO Paul Ottelini, whose company makes the processors in Macs and many Windows PCs, is one of those Mac-with-Parallels users.

VMware also has virtualization offerings for the Mac, but those options are designed more for the enterprise . Boot Camp, Apple's own software for Mac OS X, allows Windows to boot on Macs.