Then you should check out Jenkins (a open source automation server) and integrate the PowerShell plugin (as already mentioned here). Then use the RESTful API from Jenkins and let a BPMN Workflow Engine (like Activiti) trigger that. Additional to that you could connect your Human Resources Management System (HRMS) to the BPMN Workflow Engine. Via that construct it would be possible to automatically create a user based on the HRMS for example inside Microsoft Exchange (via the Jenkins Powershell integration).
A blog reader just mentioned by email that the same could be done via CruiseControl. So if you need an alternative solution pick that, but Jenkins would be the better solution I think.