Apple - How can I reinstall Mac OS X (Lion) without erasing data?

Reboot your Mac. When you hear the boot chime, hold the COMMAND + R keys. You will enter the Lion Recovery environment. From there, simply choose to reinstall Lion. It will simply remove the current system and replace it with a fresh copy. None of your data will be deleted or moved. The only thing that will be replaced will be the core system files. There is no longer any need to make backups, migrate your data, or mess with partitioning.

If you'd like to read more about the Lion Recovery function, go here: http://support.apple.com/kb/HT4718