You might try reinstalling the 10.5.6 update (from the command line) using the combo update.
After checking your boot volume with DU (using leo dvd), log in as administrator, be sure no
unnecessary programs are running. I created a bare bones admin account just for that.
1. download the 10.5.6 combo update .dmg file
2. double click to mount it, drag the MacOSXUpdCombo10.5.6.pkg out on the desktop
3. close the .dmg file and all other programs.
4. open terminal.app
5. enter the following command:
sudo installer -verboseR -pkg ~/Desktop/MacOSXUpdCombo10.5.6.pkg -target / -lang en
(press return, enter your admin password, press return again)
6. after completion, restart your computer.
Kj