Complex sentence: Modification of the Default User profile should ideally be done from new, prior to any users logging-on to the computer; if a user has already logged on once or more, the Default Profile has no effect whatsoever for that user.
Split into simpler sentences: Modification of the Default User profile should ideally be done prior to any users logging-on to the computer. If a user has already logged on once or more, the Default Profile has no effect whatsoever for that user.