Users of Microsoft Windows Vista might run into the following problem. The issue is not limited to either and can occur on both, 32bit and 64bit configurations.
The problem can occur when a user applies the Autodesk Design Review 2009 SP, effectively hiding the main toolbar - as shown below. To reiterate - the problem does not disable the toolbar, it only hides it temporarily.
You can workaround the trouble spot by clicking the restore down button at the top right or by invoking the restore command by right clicking on the title bar. The problem disappears as well if you simply minimize the application window and restore it.
Once the toolbar has been restored it will not hide again.