When I bought our house the floor vents weren't secured either.  I had a
friend with a drill put screws in each corner into the floor.  I 'think'
one at each end instead of all corners would work well.  Oh, and don't
use real long screws cause they take forever to get out when you want to
clean the vents, but do make sure they are long enough that they hold
well.  Unless you look closely you won't see the screws in the edging of
the vent covers.
 
Hugs to all. tle
Troy Lynn Eckart
Ferret Family Services
http://www-personal.ksu.edu/~sprite/ffs.html
http://www.geocities.com/Petsburgh/Haven/5481/
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[Posted in FML issue 3555]