unjoin domain & uninstall configmgr client in task sequence

Hi there,

because of several reasons i tried to setup a task sequence for our windows 7 OSD to

1. unjoin the domain (=join workgroup) with a powershell-script and

2. uninstall configmgr client.

so here is my script for unjoining the domain:

$WID=[System.Security.Principal.WindowsIdentity]::GetCurrent();
$WIP=new-object System.Security.Principal.WindowsPrincipal($WID);
$adminRole=[System.Security.Principal.WindowsBuiltInRole]::Administrator;
If ($WIP.IsInRole($adminRole)){
}else {
  $newProcess = new-object System.Diagnostics.ProcessStartInfo 'PowerShell';
  $newProcess.Arguments = $myInvocation.MyCommand.Definition
  $newProcess.Verb = 'runas'
  [System.Diagnostics.Process]::Start($newProcess);Write-Host 'Prompting for Elevation'
  exit
}
#####################
# Add Scripts Below #
#####################
$Workgroup = "private" #IF you want to add computer to domain edit here(Domain name)
$Password = "******" | ConvertTo-SecureString -asPlainText -Force
$Username = "$domain\username"
$Credential = New-Object System.Management.Automation.PSCredential($Username,$Password)
Add-Computer -WorkGroup $Workgroup -Credential $credential -Force

the script does work when executed manually but does not work in task sequence.

also i am not quite sure in which order both steps should be executed and if this is really possible!

does anyone have experience with unjoining the domain in the task sequence?