Cannot END APPLICATIONS

I am running Windows XP SP3 + all MS patches via auto-update. I am a
VERY experienced PC user.

In the last few days, my system will not shut down once it has been
running for a few hours. Nor can I end an application by "X"-ing on
the window or selecting FILE | END in the application's main window

The usual way of forcing an application to end via the Task Manager
also doesn't work.

Searching the web, there are lots of web pages devoted to Windows
Shutdown, but none apparently to Windows End Application beyond basic
instructions for using Task Manager.

Can anyone help? I hate to have to do a Windows re-install.

Thanks.