WHen I’m shutting down the 'puter for the night, Windows is hanging on the “logging off” screen or if not there, right before it.
I know there’s probably a gazillion things that could be wrong, but where (registry?) might I look to see what’s causing the hanging?
Thanks!
Take a look at the following and see if it’s of any help to you: 
http://www.computing.net/cgi-bin/AT-search.cgi?mode=concept&search=XP+shutdown+hangs+on+logging+off&forum=WindowsXP&sp=sp&x=21&y=11
Look for applications you have installed not shutting down properly first of all. (Try a clean boot:
http://support.microsoft.com/kb/310353)
After that there are many things to investigate. My old laptop used to take forever to shut down. I’m afraid I never did find out why: I think an installation of Windows just gets slower and slower to boot up and shut down as the years go by.
http://www.aumha.org/win5/a/shtdwnxp.php
http://www.windowsnetworking.com/articles_tutorials/Troubleshooting-Windows-Shutdown.html
Some considerable time ago I to had a shutdown issue with XP and I resorted to installing the Microsoft User Profile Hive Clean-up tool (uphclean.exe) mentioned in the mvps.org link above. That resolved the problem, like FWF I never did find the true cause.