Windows Server 2008 Release Candidate Evaluation Software
Windows Server 2008 RC1 Enterprise is now available with a beta version of Windows Server Hyper-V, a key feature of Windows Server 2008 that will be included in Standard, Enterprise, and Datacenter x64 editions. An earlier Customer Technology Preview CTP of Hyper-V, is also available for download with Windows Server 2008 RC1 Standard, Enterprise, and Datacenter x64 editions.
Windows Server virtualization page