Windows 8.1 ships with a new module called "Defender". The cmdlets found inside enable you to manage, view and change all aspects of the Windows Defender anti-spyware application.
To list all cmdlets available, try this:
Get-Command -Module Defender
If you do not get back anything, then you're probably not running Windows 8.1, so the module is not available.
Next, try and explore the cmdlets. Get-MpPreference, for example, lists all the current preference settings. Likewise, Set-MpPreference can change them.
Get-MpThreatDetection would list all threats currently detected (or nothing it there are no current threats).
