Software for which Operating System? If you are asking for NT based systems (NT,2000,XP), then you can go for ERUNT and NTREGOPT, a very powerful and useful tool, moreover it's free.
If it's for Windows 98, then you can use the Microsoft's Regedit itself. Go to Start > Run and type regedit and press ENTER. Here go to Registry Menu > Export Registry File. Then type a filename, and make sure that Export Range is selected to "All", and click "Save" to backup the Registry.
Wndows 98 automatically takes a backup of Registry on evry successful boot up, so that if anything goes wrong, it automatically restores the backup.
NT based systems dont have the automatic Registry backup, but it has similar feature in the form of System Restore, which restores the whole system along with Registry.
If you only want Registry backup, you can use ERUNT and NTREGOPT.