It sounds like some fonts were thrown away that are required by the system. Or the fonts have gone corrupt. The only thing you can do at that point is to reinstall your operating system. If you do a Archive and Install you will keep all of your data intact. Here is Apple's instructions on doing a Archive and Install:
Start up from your Mac OS X Install Disc 1 or Install DVD. If you have different versions of Mac OS X Installer discs (for example, if your computer came with version 10.3 and you later purchased and installed version 10.4), start from the disc that most closely matches the installed Mac OS X version. Usually, that's the latest (newer) version.
Usually, you can start from the disc by putting it in your computer, restarting, and holding the C key. Or, put it in the computer and click the Install or Restore icon you see in the disc's main window (after which the computer will start from the disc without you needing to hold C.)
Select your language. The Welcome to the Mac OS X Installer window appears.
Click Continue. The Important Information window appears.
After reading or printing the information, click Continue. The Software License Agreement window appears.
After reading or printing the information, click Continue.
An agreement sheet appears. If you agree with the license agreement, click Agree. The Select a Destination window appears.
Click the volume that you wish to install to.
Click Options. The installation options sheet appears.
Select Archive and Install.
Select the volume which already has Mac OS X installed.
You should also select Preserve Users and Network Settings, to preserve network and Home directory settings. (See notes and Related documents below.)
Click Continue to begin the installation.
Here is Apple's webpage on this information: