If you have more than one screen, you might experience the problem, that the maximum resolution available in game options is 1280x1024. Even if you have checked force desktop resolution, it will fail changing to full desktop resolution. In order to fix this behavior, you must set those values in Windows registry editor.
Important: This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
http://support.microsoft.com/kb/322756: How to back up and restore the registry in Windows
Please follow the steps as explained in this article: GGG KB1025 User Patch 1.4RC desktop resolution is unavailable