I would start with OpenWSMAN, they perform WSMAN for much more than just Intel AMT but they have samples. The other source of documentation is around Microsoft WinRM. It's a Microsoft COM object that performs WSMAN operations and so, you should be able to use it in any language that supports COM.
If you run Linux or want something small, OpenWSMAN is probably the way to go. Also note that WinRM is present in Microsoft Vista but an optional free package on XP and in both cases the user needs to configure it.