$acl = Get-Acl c:\temp
$permission = "BuiltIn\Administrators","FullControl","Allow"
$accessRule = New-Object System.Security.AccessControl.FileSystemAccessRule $permission
$acl.SetAccessRule($accessRule)
$acl | Set-Acl c:\temp

$colRights = [System.Security.AccessControl.FileSystemRights]"FullControl" 
$InheritanceFlag = [System.Security.AccessControl.InheritanceFlags]"ContainerInherit,ObjectInherit"  
$PropagationFlag = [System.Security.AccessControl.PropagationFlags]"None"
$objType =[System.Security.AccessControl.AccessControlType]::Allow 
$objGroup = New-Object System.Security.Principal.NTAccount("BuiltIn\Administrators") 
$objACE = New-Object System.Security.AccessControl.FileSystemAccessRule `
    ($objGroup, $colRights, $InheritanceFlag, $PropagationFlag, $objType) 

$objACL = Get-ACL "c:\temp" 
$objACL.AddAccessRule($objACE) 
Set-ACL "c:\temp" $objACL