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When I've installed Notepad++, the auto-completion feature didn't work when I typed CTRL+SPACE.
I found out why such a thing happens :
Messenger Plus! Live (http://www.msgpluslive.net/) is able to mask Windows Live Messenger Window when you type CTRL+SPACE. First, I didn't know it. Therefore, I disabled this function from Messenger Plus! and auto-completion started functionning.
Maybe you shoud warn people about this in FAQ.