This may be a duplicate of the bug with ID 1396987, but I had to open a new bug report, as I cannot add information there. Texniccenter seems to have a problem with negative screen coordinates under Windows (the windows version does not seem to matter, I experienced this under Win XP and Win 7). Such a situation occurs, if you have your primary monitor to the right of your secondary monitor.
If you try to open certain context menus while operating Texniccenter on the left monitor, the menus will appear on the right monitor, or with an effective size of zero pixels. Affected menus are for instance the right-click menu for the toolbar and the auto-completion window for many options (e.g. try to complete "\se"). The former will appear normally, only on the wrong screen. The latter will appear in the upper left corner of the primary monitor (location 0,0) with width and length of zero. I discovered the latter case only in Windows 7 as the thick standard borders allow you to notice the menu as "dot" in the corner.
Steps to reproduce:
1. Make a dual monitor setup in Windows, where the primary monitor is to the right of the other one.
2. Open Texniccenter and use it on the secondary monitor (the one with negative screen coordinates)
3. Right click on the toolbar or
3. Make a new tex file, type "\se" and hit CTRL-Space