Running Powershell Script To Join Domain after running Sysprep in Windows 7

I'm working on a Windows 7 image that will join the computer to the domain automatically.  I'm trying to accomplish this through a Powershell script.  While logged in as Administrator, I set the execution policy to remotesigned.  Then I use Get-Credential to Store Passwords “securely” in a file (see http://bsonposh.com/archives/338).  I create a script with the following commands:

$password = Get-Content c:\windows\system32\windowspowershell\v1.0\password.txt | ConvertTo-SecureString
$credential = New-Object System.Management.Automation.PsCredential "Domain\domainuser",$password
add-computer -domainname Domain -credential $credential af.org -passthru

I save the script in c:\windows\system32\windowspowershell\v1.0\ and I place the run script command in the Administrator's runonce registry entry.

The problem:
1. It appears that Powershell reverts back to "Restricted" mode after running sysprep thereby not allowing scripts to run automatically.
2. It seems that the Administrator account seems to change because I get an error when trying to access the encrypted password file.  I have to start running the Powershell cmdlets from scratch in order for it to work which defeats the automated process, but does join the domain.

I should note that my unattend.xml file changes the Administrator password. (Don't know if that has anything to do with it).

Please advise.