dll changes graphic aspect

Hello, everyone:

I made a dll using QT 4.8 to add functionality to another application written in C ++, but with MFC for graphics.

My problem is, when I declare in this application my dll and the screen resolution is greater than 1024 pixels, change the font, resizing the window, but with the maximize active button.

My dll uses QtCore and QtGui, but I don't change fonts or resolution (consciously).

If I do not declare the DLL on my application, this does not occur.

Any idea about this behavior?

Thanks in advance.