Well, I finally figured out what was going on when it seemed like my keyboard was deciding to go on vacation. Yesterday, I was composing a message in Outlook when I noticed that the keystrokes were not appearing on the screen. I did have a window partially exposed behind the message composing window. The partially hidden window was a remote console for a server, looking at the error log using the Microsoft Management Console snap-in. As I typed, the information in the MMC window jumped around.
I realized what was going on: the OS, for reasons of its own, was sending keystrokes to a window that didn’t have the focus. Many applications don’t respond to keystrokes in their normal state, and many windows are completely covered by others when they don’t have the focus, so I’d never noticed the behavior before. The MMC window happened to be peeking out from underneath the Outlook window, and the MMC snap-in was interpreting at least some of my keystrokes as keyboard shortcuts. The mouse continued to work properly, sending the click to the ap with the focus. I think, but don’t know for sure, that it’s always happened when I was using Outlook. Even if true, that’s not definitive; I spend a lot of time in Outlook.
I understand the problem, but don’t know how to fix it. I do have a workaround that is a lot quicker than a reboot: minimizing all the aps but the one you want the keystrokes to go to.