When installing the Qt SDK for Windows you get a set of pre-compiled Qt libraries as well as Qt Creator.
When using this Qt version for building an installer, it is important to pick the correct Qt dlls to ship with your application.
The SDK comes with two sets of dlls. One set resides in $BASEDIR/bin and the other in $BASEDIR/qt/bin. The former contains the dlls used by Qt Creator, while the latter are the dlls that you want to ship with your executable.
Thanks to Marcus for this tip!
I recently discovered this the (kinda) hard way. Thank for reminding me to actually write it down :)
Anyway you could build qt as static-linked library: this prevent mistake like wrong qt version dll or missing library :) ( But the executable become BIGGER )