Looking to identify stale/inactive computer accounts within Active Directory? Using the 'Search-ADAccount' Powershell commandlet with the 'AccountInactive' & 'ComputersOnly' parameters will help you find just what you're looking for.
Import-Module ActiveDirectory
$DaysInactive = 90
$InactiveDate = (Get-Date).Adddays(-($DaysInactive))
$inactiveComputers = Search-ADAccount -AccountInactive -DateTime $InactiveDate -ComputersOnly
