Windows Web Services API team has released the beta version of the API for Windows XP, Windows Vista, Windows Server 2003 and Windows Server 2008. It is available now on the Connect site for Windows Networking. This release contains the same version of the runtime as in Windows 7 Beta. As you can see from the table on the left, the Beta of Windows Web Services API will play nice even with Windows XP SP3 and Windows Vista SP1.
More info: Microsoft Connect