$usersid = New-Object System.Security.Principal.Ntaccount("ADDOMAIN\ad_user")
$accessRule = New-Object System.Security.AccessControl.FileSystemAccessRule($usersid,[System.Security.AccessControl.FileSystemRights]::ReadAndExecute, [System.Security.AccessControl.AccessControlType]::Allow)

Get-ChildItem -Recurse -Directory . | % { 
    Write-Host "Setting rights to $($_.FullName)"
	$fld = $_.FullName 
	$acl = get-acl $fld
	$acl.PurgeAccessRules($usersid)
	$acl.SetAccessRule($accessRule)
	$acl | Set-Acl $fld
}