In addition to Rich's excellent post, I would like you to try disabling Norton. If you don't know how to do this then don't hesitate to say so and we'll guide you through it.
I would also like you to run the System Update Readiness Tool (SURT) (by Microsoft). This will give me a better idea of what's going on
SURT download link: Download: System Update Readiness Tool for Windows Vista for x64-based systems (KB947821) [May 2012] - Microsoft Download Center - Download Details
Download and install that. When it finishes, it will generate a log file which I would like to see. The log can be found in the following location:
Double click on it to open it, then copy and paste the contents into your next post for me please
Don't delete the SURT installer after you've used it as we might need it again and it's quite a big file