$objDomain = New-Object System.DirectoryServices.DirectoryEntry
$objSearcher = New-Object System.DirectoryServices.DirectorySearcher
$objSearcher.SearchRoot = $objDomain
$objSearcher.Filter = ("computer")
$objSearcher.PropertiesToLoad.Add("name")
$Computers = $objSearcher.FindAll()
 foreach ($machine_name in  $Computers | sort computername )
{
$sql_servers = get-wmiobject -class win32_service -computer $machine_name
$sql_servers | where { $_.name -like 'MSSQL$' -or $_.name -eq 'MSSQLSERVER'} | select name
}

--Imports Microsoft.SqlServer.Management
--Imports Microsoft.SqlServer.Management.Smo
Dim myDt As DataTable = Smo.SmoApplication.EnumAvailableSqlServers(False)
Dim a() As DataRow = myDt.Select("", "[Server] asc")
--just so you can see the columns collection
For Each dr As DataRow In a
  Debug.Print(dr!Name)
  Debug.Print(dr!Server)
  Debug.Print(dr!Instance)
  Debug.Print(dr!IsClustered)
  Debug.Print(dr!Version)
  Debug.Print(dr!IsLocal)
Next