If you've been a bad nervous Mac user, then chances are that you might not be running the latest updates. If so, you can download the Combo updater for Mac OS X 10.5.5 which includes all important patches up to this point, so you can remain up-to-date even if you skipped a couple of updates.
If you are still running the slightly older OS, Tiger, then Apple has provided an update for you as well. The Security 2008-006 update allows you to stay as safe as your Leopard brethren. Security update 2008-006 is available for both PPC and Intel Macs running Mac OS X 10.4 (Tiger).
You can download all of these updates by opening Software Update (Apple menu > Software Update) or by visiting Apple's download page.