VMware, the market leader in virtualization software, says Microsoft is trying to block Windows users from implementing third-party virtualization tools. Is VMware overreacting, or is it trying to avoid being "Netscaped"--made irrelevant as Microsoft folds its own virtualization technology into the operating system?

Microsoft's VM Tech Virtual Hard Disk A virtualization file format Virtual Server Runs one virtual machine under Windows Virtual PC 2007 Allows a Windows desktop to emulate an Intel Pentium PC Viridian (code name) Will host virtual machines under Windows Longhorn Server

In a white paper on its site, VMware says Microsoft won't support Windows or its applications when they're running in a third-party virtual machine unless the user has purchased Microsoft's Premier level of support. Microsoft doesn't deny it: The Premiere requirement is part of its commitment "to providing high-quality technical support," says Mike Neil, manager of virtualization strategy.

But if Microsoft can support its Premier customers running third-party virtual machines, "there is no technical reason to deny the same to all customers," says VMware.

Microsoft's Virtual Hard Disk format lets a user create a copy of the Windows operating system and an application as a single virtual file, a combination that's ready to run in any virtual machine--theoretically. VMware says "some" of these VHD-formatted files are "configured to deactivate themselves if they find they are not running inside Microsoft's Virtual Server or Virtual PC products."

Srinivas Krishnamurti, VMware's director of product management, says it boils down to Microsoft's creating barriers to customers adopting VMware products. Microsoft's Neil didn't address the Virtual Hard Disk claim directly, but he says Microsoft's policies on its virtualization products "are progressive and fair ... creating a level playing field for partners and customers."

Netscape knows that playing field very well.