The registry is automatically backed up the first time that the system is
booted each day with this being a result of the ScanRegistry (scanregw
/autorun) entry in MSConfig | Startup.  If no rb00?.cab is being created
then this would suggest that either the PC wasn't booted on the day in
question or that perhaps the ScanRegistry entry has either been removed or
unchecked.  You can safely delete any rbbad.cab file.  This will have been
automatically created when you booted and the system had problems with the
current copy of the registry which it archived as rbbad.cab and loaded
instead a backup from the most recent rb00?.cab.

The operation of system restore has nothing to do with scanreg or the
production of the rb cab files in the sysbckup folder but is instead one
of the tasks handled by Win Me's state manager (smgr.exe).  Each system
restore checkpoint includes a copy of the registry at the time and date
the checkpoint was created.