Use System Restore, then go back and apply updates one by one until you figure out which one causes the registry to blow up.
Click Start-->All Programs-->Accessories-->System Tools-->System Restore
Note: Make sure your virus software is up to date and run a scan on the machine! You being unable to run regedit concerns me...try running regedit after the system restore to insure the problem didn't already exist. That sort of behavior stinks of virus.
...restore back to before the updates were installed. Go to Windows update and pick a custom install and install each update one by one with a restart after each one until you find out which one caused it. Once you've figured out which update it is you can use System Restore again and try uninstalling Norton before the update...if the problem goes away until Norton is reinstalled then contact Norton's support group with the name of the update that's causing the problem so they can send you an update or work on a solution with Microsoft.