• Unless you have one of the non-retail Snow Leopard discs obtained directly from Apple, there is no need to install Leopard before installing Snow Leopard.
• There is no Archive & Install option for Snow Leopard, nor does it create or need a "Previous Systems" folder. If you have one, it is left over from an A&I of Leopard or an earlier OS version.
* There is no need to reinstall anything from the "Previous Systems" folder if your prior Leopard (or whatever) system was working normally. If not, see the next two items. Otherwise, skip to the last item.
• If you did an A&I with the 'preserve users & settings' option, the only things of potential use in the "Previous Systems" folder would be system level files added by third party software
but do not add any of them back to the Snow Leopard system until you have made sure they are compatible with it. Alternately (& usually preferably), reinstall that software using its installer after Snow Leopard is installed.
• If you did an A&I but *did not* use the 'preserve users & settings' option, the home folder(s) for your user account(s) were left in the "Previous Systems" folder. See
How to get files from a previous home directory after Archive and Install for info on how to restore them. Do this
before installing Snow Leopard.
• If you have already replaced anything in the new system level folders (System, the root level Library folder, etc.) with their counterparts from the "Previous Systems" folder, you have most likely compromised the new system. This may also be true if you have added items from that folder, depending on what they are. To recover from this, just run the Snow Leopard installer again. Afterwards, do not move anything from the "Previous Systems" folder into the new system. Check for normal operation & functionality. If everything is as expected, you can delete the "Previous Systems" folder. It is no longer needed.