Users sometimes scoff or cringe at the notion of reinstalling the system. But consider this: on the one hand, system software is complicated, and the source of problematic system behavior can be difficult to track down; on the other hand, software arises in files, so the mystery is merely which files are causing the problem. Reinstalling the system is thus a guaranteed, straightforward solution, because it removes all files that can affect system behavior, starting you afresh with a completely clean, vanilla system. Users whose computers are misbehaving should not be reluctant to reinstall the system. It is perfectly safe, fairly easy, and not particularly time-consuming. No data is lost in the process.
(Of course, if your trouble is caused by hardware, reinstalling the system probably won't help. But this very fact can be a useful diagnostic, helping to prove that the trouble is hardware, not software.)
Record user data Start by noting the names and (if known) passwords of your computer's user accounts, so that you can restore them later. Also, note any network configuration information needed for connectivity.
Assess free space Make sure your hard disk has lots of free space (more than 10GB). If there isn't room on your hard disk for a new clean system in addition to everything that's there now, along with plenty of scratch space, Archive and Install isn't even an option.
Locate and start up from your system installer disk You may be able to reinstall using an Install or Software Restore disk that came with your computer. The description that follows, though, assumes you're using a stand-alone system installer disk (for Tiger, it's a DVD). Insert the disk and start up from it. Begin performing the installation.
Specify a clean Archive and Install When you get to the "Select a Destination" screen, after specifying the volume containing the system you want to replace, press the Options button. In the resulting dialog:
Choose "Archive and Install". This means that your entire existing system, including applications, and all users and their home directories, will be moved off into a secondary folder, and a new, completely clean system will be created without deleting anything.
Uncheck "Preserve Users and Network Settings". This is important! The checkbox is offering to automatically meld some of your existing preferences and other settings files into the new system. But such a meld risks incorporating into the new system the very files that may be corrupted and causing the trouble. You want the installation to be truly clean.
Recreate users After the installation is complete and you've rebooted into the newly installed system, you're asked to enter your user information. Do so, recreating the first admin user from before. When you reach the Finder, use the Accounts pane of System Preferences to recreate any other previously existing users.
Prepare to migrate your stuff You are now in a completely vanilla Mac OS X world. All old stuff appears to be missing, but it isn't; it is all in a top level directory called Previous Systems, probably in a subdirectory called Previous System 1. Here you'll find your old Applications folder, for example, and in the Users directory you'll find the home directories of all previous users, complete with all their data.
The idea now is to copy those applications and that data from their place in the Previous Systems directory into the corresponding place in the newly installed, clean system. You may encounter permissions restrictions that prevent you from doing this easily in the Finder. If so, there are two simple solutions:
Restart the computer in FireWire target disk mode and connect it to another computer. Using the Finder's Get Info dialog on the target volume, set it to ignore permissions there.
Alternatively, create the root user on your computer; then log out and log back in as the root user.
Either way, you now have an unrestricted ability to move files from one place to another on the target volume.
(If you're a power user with material that may need restoring from some invisible location, such as the old /usr directory, you may also need the unrestricted ability to see files and folders, even normally invisible ones. An easy approach is to use TinkerTool, which has an option to make the Finder show invisibles. Or you can use the command line in Terminal to view and copy material of this sort.)
Migrate your stuff The idea now is to move or copy your old stuff into the corresponding place within the new, clean Applications directory, User home directories, and so forth. Here are some tips:
Be careful not to overwrite any existing system-created folders! For example, do not copy your entire old Preferences folder into your new Library folder; rather, copy individual files from within your old Preferences folder into your new Preferences folder.
Be careful not to overwrite any existing system-created files. For example, don't copy Safari from your old Applications folder into the new Applications folder, because there is already a clean Safari in the new Applications folder. In a few cases, if you really know what you're doing, you might break this rule; for example, you might copy Library/Safari/Bookmarks.plist from its old location into the new one, even if a Bookmarks.plist is already there, because your old bookmarks count as a piece of your old data that you don't want to lose.
Be thorough, but don't overdo it. Files can lurk in unexpected places, so you should be fairly systematic in your exploration of the old system. But don't feel that you have to copy absolutely everything. Preference files, for example, will be recreated anyway when you start up the corresponding application. You might later have to re-enter a serial number when you start up an application for the first time; you might even have to reinstall an application from scratch, so that it places its pieces in the right places. But this is not difficult and can be dealt with as the need arises.
Remember, the goal here is to keep the new system lean and mean, and not to accidentally migrate any of the files that might be causing the trouble in the first place. So confine your migration to the essentials. From time to time during the migration process you might like to pause, log out, and log in as an ordinary user, just to make sure the new system is still working correctly.
Clean up After migration, you should probably remove the Previous Systems directory, so that applications within it won't accidentally start up. If you think that the Previous Systems directory may still hold neglected data you might wish to retrieve later, use Disk Utility to copy it into a disk image; then unmount the disk image and delete the Previous Systems directory. This ensures that the whole previous system is retained, yet not directly accessible.
Start up as a normal user and make sure that all is well. If you enabled the root user, you might want to disable it again, to prevent its misuse.
In a few cases, it may be that permissions resulting from moving files have not resolved themselves properly. So some permissions may have to be manually readjusted. This problem, however, should not normally present itself, and explaining how to fix it is outside the scope of this discussion.
As mentioned earlier, you may discover over time that some third-party applications may not work properly after migration; serial numbers may have to be re-entered, or the application may have to be reinstalled from scratch. Such discoveries are easily dealt with on a case by case basis.
Finally, you'll want to update the system. Once again, the goal is not to reintroduce whatever caused the trouble in the first place, which in some cases means avoiding the update(s) that were applied directly prior to the problematic behavior. For instance, if you started experiencing issues after updating to Mac OS X 10.4.9, you may want to use the Mac OS X 10.4.8 combination updater available from Apple's download page and avoid Mac OS X 10.4.9 altogether.
In addition, you might like to reread our suggestions for how to perform a system update.