Backing up & Restoring

What is the proper way to backup and restore the vital files for a win-98 os. I have the windows backup utility, but am not sure about the files to backup other than the system.dat & user.dat files.
THE REASON: Sometimes a computer is missing one or two files, and wont boot-up. At this point the error might be vague and not very helpful, so it would be nice to be able to get it going by the backup.
1)So, what would you backup..where would you back it up, and how would you back it up to get a corrupted os working again?
Thanks…………Pat