mysterious (encoding?) problem

[guanxi <guanxi_i@NyOahoSo.cPAom.TLD>]

I hope someone can solve this mystery.  I suspect I'm hitting an odd key 
combination, but can't figure it out.

Occasionally, maybe every other day, I run into this odd problem:

No matter what letter I type, lower or uppercase, Emacs inserts a 
question mark "?".  My hex editor and other text editors shows them as 
question marks, so it isn't a display issue.

Other programs work fine.  Exiting that instance of Emacs (even leaving 
other instances open) and loading a new instance with the same file 
fixes the problem.


More mysterious details:

   * The numbers, 1-0, come out as, in order:
!??§%:.;,/

   * Shift+[1-0] outputs the numbers themselves:
1234567890

   * M-f is interpreted as C-M-f (which my Windows hotkey to start Firefox).

   * M-> (end of buffer) is ignored -- nothing happens.

   * Most standard C- commands work.  So while typing "n" outputs "?", 
typing C-n navigates normally.

   * IIRCC, with multiple instances of Emacs open, only one is typically 
affected.  The other instances work fine.

   * Brief experience with the minibuffer suggests it behaves the same.


Thanks in advance to anyone who can help.  It's not the end of the 
world, but it's annoying.

guanxi