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
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