After years of delays and billions in development and marketing efforts, it would seem that Microsoft Corp. would want anyone who possibly can, to buy its new Windows Vista operating system. Yet Microsoft is making it hard for Mac owners and other potentially influential customers to adopt the software.

Microsoft says the blockade is necessary for security reasons. But that is disputed. The circumstances might simply reflect a business decision Microsoft doesn't want to explain.

The situation involves a technology known as virtualization. Essentially, it lets one computer mimic multiple machines, even ones with different operating systems. It does this by running multiple applications at the same time, but in separate realms of the computer.

Virtualization has long been used in corporate data centers as a way to increase server efficiency or to test programs in a walled-off portion of a machine. The technology also has been available for home users, but often at the expense of the computer's performance.

But now that Macintosh computers from Apple Inc. use Intel Corp. chips, just like Windows-based PCs, virtualization programs let Mac users easily switch back and forth between Apple's Mac OS X operating system and Windows. That could appeal to Mac enthusiasts who want access to programs that only work on Windows, including some games.

Consequently, the launch of Vista seemed to be a good opportunity for Parallels Inc., a subsidiary of SWsoft Inc. that sells virtualization products.

Unlike Apple's free Boot Camp program that lets Windows run on a Mac, Parallels' $80 virtualization product for Macs does not require users to have just one operating system running at a time. Parallels runs Windows in a, well, window on the Mac desktop.

Parallels also sells a $50 version for Windows PCs — which would let people run both Vista and its predecessor, Windows XP, so they can keep programs that aren't yet Vista-compatible.

The price of the virtualization software does not include a copy of Windows. And to get that copy, buyers have to agree to Vista's licensing rules — a legally binding document. Lurking in that 14-page agreement is a ban on using the least expensive versions of Vista — the $199 Home Basic edition and the $239 Home Premium edition — in virtualization engines.

Instead, people wanting to put Vista in a virtualized program have to buy the $299 Business version or the $399 Ultimate package.

Macs account for less than 5 percent of personal computers in the U.S., but Ben Rudolph, Parallels' marketing manager, says they nonetheless represent a market he's surprised to see Microsoft present with roadblocks.

''Vista is undeniably cool and undeniably important,'' Rudolph said. ''This is really an opportunity to reach people who normally wouldn't be using Windows, whether it would be Mac users or Linux users.''