Why do you want to reinstall system software? Perhaps your Mac is unstable, no matter what you do. At this point, all your efforts to clean things up have gone for naught.
There is a drastic method to fix everything, but it's not something you would do normally, and that's to run your Mac Restore CD (or CDs, because some models come with several). When you do that, however, you might lose all your custom program settings, and (if you opt for the erase disk option), all the files you created on your Mac. What's more, if you have updated your Mac Operating System, all that will be lost as well. So I mention it here as an option, but only as a last resort.
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Reinstalling Mac OS X does not necessarily replace your system accounts, information, or configuration. There are, however, a few drawbacksmost notably, the system updates are replaced by the original version of the operating system. After running the Mac OS X Installer to recover a damaged system, you must force an update on your computer by going to Software Update and clicking the Check Now button.
Here are the steps needed to reinstall OS X:
Using Your System Installer CD
Get out your system Install CD, press the CD button, and insert the CD in your CD drive.
Restart your Mac. If need be, force a restart as described previously.
As soon as you hear the computer's startup sound, hold down the C key. This will enable your computer to start from your system CD.
The installer will launch automatically.
After your system installation is done, go ahead and restart and check that everything is working properly.