Change ownership of all files, from one owner to another
As starter I would suggest you run the "repair permission" function in disk utility.

If the problem still persists you could run a command like:

find . -user old_user | while read file; do echo chown new_user  $file; done


note: this doesn't do anything as is now - you will need to adjust it to your needs - and make sure it does nothing bad.

update learnt some more and now I would run a command like this:

find . -user old_user -print0 | xargs -0  sudo echo chown new_user


again adjust the command before you run it!