I've been researching this for the last 45 min. It seems to only be an incompatibility between non-current versions of Norton and the SP3 update process. Symantec has a bulletin about it, and it appears that as long as you go through the Symantec Live-Update to get current, it's not an issue. Yes it looks like the registry fix is hit or miss for those that experienced the problem pre-Norton update.
That being said, and regardless - Norton is the devil. The software misses a lot, it hogs your system for resources and costs way too much. For better (and free) virus protection, I only recommend AVG. It gets a lot more stuff, it's faster, and doesn't god you down. We geeks have been using it for many years.