You already have the XML template in a string so there’s no need to save it to disk and then load it as xml. It’s much easier to simply cast it as xml:

[codesyntax lang=”powershell” lines=”normal”]
$xml = [xml]$template
[/codesyntax]

As for setting the properties in XML, I don’t like hardcoded arrays since it makes the code more difficult to read (and I do read a script before executing it). It’s also more error prone as the order might change in the future.

So instead of this:
$xml.unattend.settings[0].component[1].UserData.FullName = $RegisteredName

I recommend this:

[codesyntax lang=”powershell” lines=”normal”]
$ns = New-Object Xml.XmlNamespaceManager($xml.NameTable)
$ns.AddNamespace(“unattend”, $xml.DocumentElement.NamespaceURI)

$msWinSetup = $xml.SelectSingleNode(“//unattend:settings[@pass=’windowsPE’]/unattend:component[@name=’Microsoft-Windows-Setup’]”, $ns)
$msWinSetup.UserData.FullName = $RegisteredName
$msWinSetup.UserData.Organization = $Organization

$msWinShellSetup = $xml.SelectSingleNode(“//unattend:settings[@pass=’specialize’]/unattend:component[@name=’Microsoft-Windows-Shell-Setup’]”, $ns)
$msWinShellSetup.ComputerName = $ComputerName

$msWinUnattendJoin = $xml.SelectSingleNode(“//unattend:settings[@pass=’specialize’]/unattend:component[@name=’Microsoft-Windows-UnattendedJoin’]”, $ns)
$msWinUnattendJoin.identification.JoinDomain = $DomainName
$msWinUnattendJoin.identification.credentials.UserName = $DomainUser
$msWinUnattendJoin.identification.credentials.Password = ConvertTo-PlainText $DomainPwd
$msWinUnattendJoin.identification.credentials.Domain = $DomainName
[/codesyntax]