Something funny with fonts...?

I wonder if there's something strange going on with Microsoft Updates or
something. My wife's XP machine and my Vista/64 system both are having
font-loss problems concurrently... and we don't even share a network. Those
fonts in the list with the image of an 'O' are unaffected, but older 'TT'
(TrueType) fonts are disappearing from the system font list. We store all
our fonts in another folder, and I can point the font installation utility to
a font in that folder and click Install. Then I get a message that the font
is already there, and sure enough, when I go back to check it's there. Then
Word or whatever can find it fine. Anyone know what's going on?