VMware, whose virtualization software helps IT departments maximize use of their server hardware, said Wednesday that its product will run reliably with software from Microsoft.

VMware has certified its ESX hypervisor to work with Windows Server and other software from competitor Microsoft. The move also gives customers using ESX technical support from both companies to deploy VMware virtualization software on Microsoft infrastructure.

ESX update 2 is the first hypervisor to be certified through the Microsoft Server Virtualization Validation Program, introduced by Microsoft in November 2007, according to VMware.

The program enables vendors to test and validate virtualization software running on Windows Server 2008 and previous versions of the OS. It also enables Microsoft to offer cooperative technical support to customers running Windows Server on validated virtualization software other than Hyper-V, its own hypervisor.

VMware remains the leader in selling virtualization software, which allows multiple and different OSes to run on one piece of server hardware. According to IDC, VMware had 76.4 percent market share in 2007, followed by IBM with 9.8 percent.

However, competitors -- particularly Microsoft -- are hoping that won't be the case for long. Microsoft released Hyper-V earlier this year as a competitive offering to ESX and the hypervisor of choice for Windows Server. At the same time, Microsoft also realizes it must work with other virtualization vendors, which is why it introduced the validation program last year.

VMware has already been feeling the effects of competition from Microsoft and other vendors seeking to commoditize the technology on which VMware's business was built. In July, the company replaced its President and CEO Diane Greene with a former Microsoft executive, Paul Maritz. Her departure came as the company lowered its revenue forecast for fiscal 2007.

On Tuesday, VMware disclosed that its Executive Vice President of Research and Development Richard Sarwal was leaving for his previous employer, Oracle, after less than a year at VMware.