How can I use Active Directory commands in a Powershell script?
They are available in Server 2008 R2 as an installable feature. I believe Server 2008 R2 is the only platform that supports that.
I would highly recommend installing Quest Software's free set of Active Directory cmdlets.
With this, you get access to commands like Get-QADUser and Get-QADObject which probably provide all the functionality you will need, without the dependency on Active Directory Web Services that Get-ADUser has.
Another option would be to use PowerShell's ability to instantiate .NET objects and use the DirectoryServices namespace of the .NET Framework. But unless you need something that is not available via the Quest tools, using .NET is probably more complicated than you need.