Problems opening programs

Hi folks,

I have a Power MAC G5.
Since a couple of days ago some programs like iTunes, Safari, Textwrangler, etc wouldn't open. The most weird part of it is that some programs open and others don't but I can't find I link between them. Strange!
The problem seems to have started after I allowed the Software Update to download and install some updates.
I've tried everything but can't find the solution.
Do you guys have any idea of what could be causing it?
Thanks a lot.

could be a permissions error, try that first

Run disk utility to repair permissions

I've figured it out. I have a friend who had a look at it for me.
It had something to do with the WEBKIT. Somehow the file was missing and all the programs with some conection with the net was not starting up. We had an old version of WEBKIT, so we just reinstalled it.
Now it seems to be working fine.
Thanks anyway for the help Louishen.