Open the client (illarion shortcut). You'll have this window:
Click on "options", this window will appear:
In each of the tabs, make sure you have the same settings as me.
Then go to your desktop, right-click on it and select properties:
In the "settings" tab, make sure your resolution is high enough. Try putting it on the max, or at least put it on anything equal to or higher than 1024 x 768