Microsoft has released Windows Server 2008 evaluation VHD (Virtual Hard Drive) images for Hyper-V which as of now is available for download from Microsoft 
Download Center. VHD format is the common virtualization file format used by virtualization software’s from Microsoft like Virtual PC, Virtual Server 2005 and Hyper-V that provides a uniform product support system, and provides more seamless manageability, security, reliability and cost-efficiency for customers.
These images will require Hyper-V and for that you will need to have a base install of Windows Server 2008 (64bit edition) with Hyper-V running. For more information on obtaining and installing the latest version of Hyper-V, please visit the Hyper-V Homepage.
This release is intended for testing purposes only and can be evaluated for 60 days. However, this 60 days evaluation limit could be increased to 240 days through re-arm method provided by Microsoft. After this time, you will need to uninstall the software or upgrade to a fully-licensed version of Windows Server 2008.
Further instructions on how to install Windows Server 2008 Evaluation VHD images with Hyper-V can be found here.
Download Windows Server 2008 Evaluation VHD Images for Hyper-V
