The link provided by Mark no longer works, but you can go to:
Notpad++ 6.6.9
- Settings -> Preferences -> Auto-Completion -> Enable auto-completion on each input.
I find it very annoying though, since a big autocomplete block is always coming up and I would just like to see autocomplete when I press tab or a key combination. I am fairly new to Notepad++ though. If you know of such a key combination, please feel free to reply. I found this question via Google, so we can always help others.