Could be many things, we should start with this...
"Try Disk Utility
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair. Disk Utility checks and repairs the disk."
Report any errors found & if they were repaired.
Likely you'll have to re-install.
Some general advice on updating:
It is worth noting that it is an extreme rarity for updates to cause upsets to your system, as they have all been extensively beta-tested, but they may well reveal pre-existing ones, particularly those of which you may have been unaware. If you are actually aware of any glitches, make sure they are fixed before proceeding further.
So before you do anything else:
If you can, make a full backup first to an external hard disk. Ideally you should always have a bootable clone of your system that enables you to revert to the previous pre-update state.
Turn off sleep mode for both screen and hard disk.
Disconnect all peripherals except your keyboard and mouse.
1. Repair Permissions (in Disk Utility)
2. Verify the state of your hard disk using Disk Utility. If any faults are reported, restart from your install disk (holding down the C key), go to Disk Utility, and repair your startup disk. Restart again to get back to your startup disk.
At least you can now be reasonably certain that your system does not contain any obvious faults that might cause an update/upgrade to fail.
3. Download the correct version of the COMBO update from the Apple download site.
The Combo updater of Leopard 10.5.8 can be found here:
If you prefer to download updates via Software Update in the Apple menu (which would ensure that the correct version for your Mac was being downloaded), it is not recommended to allow SU to install major (or even minor) updates automatically. Set Software Update to just download the updater without immediately installing it. There is always the possibility that the combined download and install (which can be a lengthy process) might be interrupted by a power outage or your cat walking across the keyboard, and an interrupted install will almost certainly cause havoc. Once it is downloaded, you can install at a time that suits you. You should make a backup copy of the updater on a CD in case you ever need a reinstall.
Full details about the 10.5.8 update here: http://support.apple.com/kb/HT3606
More information on using Software Updater here:
Using the Combo updater ensures that all system files changed since the original 10.5.0 are included, and any that may have been missed out or subsequently damaged will be repaired. The Delta updater, although a temptingly smaller download, only takes you from the previous version to the new one, i.e. for example from 10.5.7 to 10.5.8. Software Update will generally download the Delta updater only. The preferable Combo updater needs to be downloaded from Apple's download site.
Now proceed as follows:
4. Close all applications and turn off energy saving and screensaver.
5. Unplug all peripherals except your keyboard and mouse.
6. Install the update/upgrade. Do not under any circumstances interrupt this procedure. Do not do anything else on your computer while it is installing. Be patient.
7. When it ask for a restart to complete the installation, click restart. This can take longer than normal, there are probably thousands of files to overwrite and place in the correct location. Do nothing while this is going on.
8. Once your Mac is awake, repair permissions again, and you should be good to go!
If your Mac seems slightly sluggish or ‘different’, perform a second restart. It can’t hurt and is sometimes efficacious! In fact a second restart can be recommended.
9. Open a few of your most used applications and check that all is OK. In this connection please remember that not all manufacturers of third party applications and plug-ins, add-ons, haxies etc, will have had time to do any necessary rewrites to their software to make them compliant with the latest version of your operating system. Give them a week or two while you regularly check their websites for updates.
N.B. Do not attempt to install two different updates at the same time as each may have different routines and requirements. Follow the above recommendations for each update in turn.
Lastly, Apple's own article on the subject of Software Update may also be useful reading:
[b]If you are updating Safari (or just have):[/b]
Input Managers from third parties can do as much harm as good. They use a security loophole to reach right into your applications' code and change that code as the application starts up. If you have installed an OS update and Safari is crashing, the very [i]first[/i] thing to do is clear out your InputManagers folders (both in your own Library and in the top-level /Library), log out and log back in, and try again.
So, disable all third party add-ons before updating Safari, as they may not have been updated yet for the new version. Add them back one by one. If something goes awry, remove it again and check on the software manufacturer's website for news of an update to match your version of Safari.
Most errors reported here after an update are due to an unrepaired or undetected inherent fault in the system, and/or a third party add-on.
Additional tips on software installation here:
To reiterate, Input Managers reach right into an application and alter its code. This puts the behavior of the affected application outside the control and responsibility of its developers: a recipe for problems. That's not to say that issues absolutely will ensue as a result of Input Managers, but you, as a user, must decide. If the functionality of a specific Input Manager or set thereof is really important to you, you may well choose to assume the associated risk.
Again, the advice is to remove all Input Managers from the following directories:
especially prior to system updates (they can always be added back one-by-one later).
Solutions for troubleshooting installation, startup, and login issues in Mac OS X v10.5