When I mouse-drag a window's titlebar, the window doesn't move.
When I alt-space, m, arrow keys, the window does move (and then mouse-drag works, until I click and release the mouse button.)
Same problem with window resizing: mouse-dragging the edges or corners does nothing until I alt-space, s, arrow keys.
sfc /scannow, but when I ran that, it "did not find any integrity violations."
Applications run fine (file explorer, web browser, media players). Other mouseable window-decorations work fine.
This misbehavior started overnight, without a reboot.
Since then, it's been unaffected by several reboots, and a windows update (just 2 things, for .NET framework). It even happens when booting into Safe Mode. It also happens with other accounts that I create to login to.
Is there a less harsh way to fix this than by reinstalling Win7?
[Edit, Feb 17 '14:] Commenter Daniel Wolfe asked if I'd found a fix. The misbehavior went away by itself, as mysteriously as it came, the same day (see comment "good grief"). It hasn't recurred. Perhaps the answer to my question is: wait and hope.