If you have installed latest SDK for Vista (http://www.microsoft.com/downloads/details.aspx?FamilyID=117ecfd3-98ad-4d67-87d2-e95a8407fa86&DisplayLang=en) and trying to write an applications for Windows PowerShell (http://www.microsoft.com/windowsserver2003/technologies/management/powershell/default.mspx) you may be looking for System.Management.Automation.dll assembly.

 It is located in c:\program files\Reference Assemblies\Microsoft\WindowsPowerShell\v1.0