Microsoft Knowledge Base Article 867572 contains the following intoduction:
This article describes the guest operating systems that are supported by Microsoft Virtual Server 2005. Additionally, this article describes the host computer operating systems that you can install Virtual Server 2005 on.
Note As of June 2004, Virtual Server 2005 requires a 32-bit host computer operating system.
Additionally, you cannot install Virtual Server 2005 on a computer that is running a 64-bit edition of Microsoft Windows.