Brute force....
Download the OS X client 10.6.8 combo update or the OS X Server 10.6.8 combo update directly from Apple, install that, and see if the problem clears. (If you don't know what OS X Server is, you're probably not running it.)
If applying the combo update fails to resolve this, try running a disk verification and a disk permissions repair, then re-try the combo update, and see if the problem clears.
If the above doesn't clear the issues, then you'll want to check the logs using Console.app and see if there are any errors related to why the updates are not being applied.