Do it like this:
- Restart your computer.
- Find the Disk Utility (in the Utilities folder, in the Applications folder, on your hard disk) and “repair permissions.” If you don’t know how to do this have me do it with you over the phone one time.
- Download and install the 10.5.6 COMBO update (this is better than going to the Apple menu and choosing Software Update.)
The updater will want you to restart when it’s done. Do that, then run Disk Utility again, and repair permissions again. That’s it.
(The updater is full of bug fixes, including some for Mail and iChat. You won’t notice much new but your machine will run better after the 10.5.6 update.)