It is very easy, from within OS 9, to erase vital OS X system files. There are no file permissions preventing you form doing so.
I doubt you need a full reformat ans reistall. You can probably just reinstall OS X from the install CD. If you only have restore CD's, there (at least there used to be) an option to restore w/o erasing data.
I would back up at least you most important data berfore attempting.