What you are seeing is that the font file for the system font has been corrupted. Since the font file is corrupted then the individual letter glyphs cannot be loaded and so you are getting the symbol you are seeing.
There are a couple of reasons why the font file may have been corrupted. Assuming that you have not done anything that would have modified system files then the most likely cause is a bad disk block. You could use something like DriveX to verify all your disk blocks and then copy the font file back into place but that requires a lot of messing about. The easiest way to fix this is to copy your data off the machine, boot from the install CD, launch Disk Utility and erase *and verify* the disk, then reinstall OS X. (Note that a full erase and verification of the disk will take a very long time.) You may also want to use DriveX or a similar disk block checker after you format the drive to ensure that the bad block(s) are marked bad in the block map.