Lizard,
before we go into a lengthy repair procedure (which does not even guarantee success) you should do an Archive&Install:
1. Check for a minimum of 4 GB of free hard disk space on your hard disk
2. Unplug all peripherals
3. Insert the Install DVD and reboot while holding down the c-key
4. Once you are booted go to the menu and select disk utility
5. Repair the disk and
Repair permissions!
6. Quit disk utility and proceed with the installation
7. Under "options" select "Archive&Install" and check the box "keep user and network data".
8. Install Tiger
9. Restart
10. Go to the folder Application/Utilities and run disk utility to repair permissions again.
11. Download (or open if you downloaded before)
Mac OS X 10.4.8 Combo Update (PPC) and install it.
12. Restart and repair permissions
13. Go to the Apple menu and select "Software Update"
14. Install all missing updates and restart.
15. Repair permissions
Good luck!
If this answered your question please consider granting some stars:
Why reward points?
P.S. In the future you can save appr. 500-800 MB of disk space by running
Monolingual to remove all languages you do not need.