I\'ve downloaded Qt 4.8.2 library, Qt Creator 2.5.2, and manually installed MingW with w32api version 3.13 and GCC/g++ version 4.7.0.1. My OS is Windows 7 Ultimate x64.
I also got this problem. I'm a Qt n00b and tought, when installing Qt-libraries, that "well I already have Mingw installed so I skip installing the Mingw that comes with Qt". That gave me prolems. When installing mingw that came with Qt everything worked ok.
So my advice to anyone googling to this question (like I did) is to instead of using your already installed Mingw, install the one with Qt and use that (otherwise you have to build the Qt libraries within your Mingw, like the answer from Michael Burr)