Would also recommend a clean install, or at least an archive and install instead of the upgrade.
Also, a clean install will do nothing to your Bootcamped partition as long as you don't mess with it during the install. My Bootcamp Assistant died a month before Leopard came out, and still was able to boot into windows during that time and after a clean install of Leopard.
Anyone with better ideas, can of course jump in at any time.
And you could try something I try almost everytime I have any issue now. Create a new user and check to see if it is system wide or not. Still, some issues I had related to my user account under Tiger were never repaired until I installed Leopard. I just put up with logging in to the other account.