RegRestoreKey

I am having problems using the Win32 API RegRestoreKey. I have a program which backs up portions of the registry to hive files, using the RegSaveKey API. I need to be able to restore these at a later date.
I am using NT4 SP3, so the RegRestoreKey API call is available. I have also changed the priveleges of the process to allow the restoration of registry keys. To no avail. Each time I debug the call the return code is always Access Denied.

Any help from someone who has used the API call successfully would be greatly appreciated.

Thanks in advance.