Windows image from KUKA is not only a standard Windows but already contains non standard software needed to run a KSS on top. This among other stuff includes the robots realtime os VxWorks the needed user accounts for VxWorks and necessary device drivers. So to get back to the complete factory default also reinstalling kuka windows is needed. But sometimes it is sufficient to reinstall kss as mentioned above. You could try it. I am not sure this is sufficient for safe operation. Before doing this you should of course check whether all needed installers for add-ons like safe robot are available. Just in case having the needed windows images never hurts. This is stuff the seller should always provide (but honestly often is not included 😈). In that case you can always contact Kuka. If you provide the robot serial number and there exists a valid license for the respective addon they are happy to send you the software.