Can you do a before and after of the process list and list the differences? Just after the first shutdown and just before the second shutdown attempt.

I'd agree use a couple of registry cleaners -- CCleaner & Registry Mechanic are particularly good.
Have occasional similar problem, that the PC won't shut down. Found it is an app that doesn't close properly - even one that has been uninstalled. As you've uninstalled Norton, did you use symantecs removal tool, or just add/remove?

Good point ... I did use the Add/Remove tool. However, do you know where I can get hold of the Norton removal tool?

You can get it from here...

http://service1.symantec.com/SUPPORT/tsgeninfo.nsf/docid/2005033108162039

I have now tried this but no improvement. I can't even log my processes to display what difference there is between the two attempts to shut down!