To rename a computer when Rename-Computer is not available (pre Powershell 4.0)

Get-WmiObject Win32_ComputerSystem -ComputerName OLDNAME -Authentication 6 |
ForEach-Object {$_.Rename(“NEWNAME”,”PASSWORD”,”USERNAME”)}

You can also mask the password

$credential = Get-Credential
Get-WmiObject Win32_ComputerSystem -ComputerName OLDNAME -Authentication 6 |
ForEach-Object {$_.Rename(“NEWNAME”,$credential.GetNetworkCredential().Password,$credential.Username)}