Being a built-in component of the OS, you generally DON'T reinstall it; you can probably extract a fresh copy of it from the OS installer manually, then reapply the 10.5.2 combo update to make sure it's at the latest version, but you'd probably be better off doing an archive and install on the whole OS.
But a better question is: Why do you want to reinstall it? Is it doing something odd (which a reinstall probably won't fix), or did you accidentally delete it?