Virtual Server 2005 previously was licensed at an extra fee. Now it is a free download, usable on any Windows Server 2003. You can also install and run it on Windows XP, but that is not supported for production use.
or Go to the Virtual Server 2005 R2 home page.
Also, in the interop department, in case you missed it,
MSVS2005R2 will host Linux guests. This was always possible,
but now Microsoft is supporting it.