But VMware's rivals don't today support Windows. Microsoft Corp., which has its own Windows-only virtualization product, Virtual Server 2005, prevents paravirtualization vendors from modifying Windows.

Meanwhile VMware, through its full emulation, already supports Windows and other OSes, XenSource, Virtual Iron and others have to rely on the new Intel or AMD chips with built-in VT to offer Windows. That means they need to convince would-be customers not only to move to virtualization but also to buy new hardware -- a costly move that runs counter to the virtualization market so far, said Charles King, an analyst with Pund-IT of Hayward, Calif.

"VMware has done well by managing to open up and drive the conversation on how to gain additional value from your low-value machines," King said. "As popular as Linux has become -- and is still becoming -- the market for Linux servers remains just a quarter of the size of the Windows server market. Xen and Virtual Iron are aiming big guns at a relatively small market."

Take, for example, Central Transport International Inc., which said it saved US$250,000 last year by consolidating more than 50 Windows servers into 10 by using VMware GSX and ESX Server software, according to Craig Liess, Central Transport's server administrator.

The Warren, Michigan-based trucking firm hopes to have half of its servers, including parts of the firm's imaging system and its mission-critical intranet, running virtual environments by year's end. Though Central Transport runs some Linux servers, the vast majority are Windows servers that Liess doesn't plan to swap to Linux anytime soon.