After some more investigation using google, I found this here : http://forums.x-plane.org/index.php?showtopic=88367
One solution could be applied to me. The font size of my windwos was on 150%. Changing it back to 100% PLUS logout/login (or reboot) did made it. After installation I could set set it back to 150%.
Its a bit odd, that this is not yet fixed in the installer (makeing the window user sizable is a simple tick in the window attributes). But maybe they are using any third party product.