A family of Microsoft word processing software products for creating web, email, and print documents.
I've just had this very same frustrating issue with a user I have just upgraded and found a solution that worked for her.
http://support.microsoft.com/kb/983262
Basically, if the document zoom is high so that the scroll bar at the bottom appears, Autocomplete doesn't work. I know, dumbest thing ever right.
I reduced this particular users zoom from 170% to 150% and it started working. I wasted an hour on that.