The problem turned out to be that the
compatibility mode had been set to "Windows 98/Windows Me." I found
this out after trying just about everything else possible. Disabling it
fixed the problem.
In case anybody else has this problem, right-click the mouse (assuming
your mouse is set to the default right-hand configuration) on the
PowerPoint shortcut icon (on the desktop, startmenu, or in /Program
Files/Microsoft Office), select Properties and then the Compatibility
tab, and uncheck the "Run this program in compatibility mode for:" checkbox.
That should solve it. I've tried checking and unchecking the box and
the symptoms were consistent in appearing and disappearing.
Note that this works when you have Office 97/PowerPoint 97 installed on
the XP Home Edition. I don't know if it works on other combinations.
I'd taken home from work some PowerPoint XP files a few weeks ago to
troubleshoot a merge problem someone else was having; maybe XP decided
that PowerPoint 97 needed to be in Windows 98 mode and changed it for
me. Or maybe I'd checked it and had forgotten about it.
--Bill