What system software are you running, and what kind of Macintosh do you have?
If you have Panther, then reinstalling Safari via the download from Apple's site is not an option, since that's version 1.2 and you want version 1.2.3. Try extracting it from your original installer CDs with Pacifist, then run software update or apply the 10.3.5 combo updater to make sure Safari's up to date.
If you're not running Panther, ignore all that.