It should work fine in Vista, assuming you're using the same version of Word
you were using in XP.
If you've moved to Word 2007, however, AutoComplete has been disabled due to
the enormous quantity of Building Blocks (the expanded successor to
AutoText) that would otherwise cause AutoComplete to occur just about all
the time.
To force completion after typing the first four characters, you'll need to
press the F3 key. This will work with any Building Block, not just AutoText.
But, it will work only if there are no other entries that begin with the
same 4 letters.
As a work-around, many users are finding it useful to use AutoCorrect
entries instead of AutoText entries, and to purposefully create shorthands
that match what they were typing in Word 2003 and earlier. There's still no
AutoComplete display, but typing any word separator (space, period, comma,
etc.) will trigger that automatic change from the shorthand into the
expanded form.