I have been trying to use the AutoComplete component from Adode for a few weeks now, and it has been rather painful. I keep finding problems with it, and it has been frustrating my users a lot. There is a modified version by Jens Krause that is much better.
But I still wasn't satisfied, so I started looking around for another auto-complete component, and came across this Chooser made by Hillel Coren. Check it out, it's very powerful and best of all it's freely available.