Had endless trouble with this but eventually spotted a reference for Backup Exec (a similar product) associating this error with lack of permissions for cmd.exe. In fact my permissions were correct but I had changed comspec to tcc.exe, a better command processor. Permissions for tcc.exe were already fairly full and adding TrustedInstaller made no difference, but just putting comspec back to C:\Windows\System32\cmd.exe fixed the problem - a smooth upgrade from NG14 to NG15 followed.
Hope that helps someone!