A while back we had the same problem. It turned out to be a small virus. Run your scan and see if that helps. Or I you are using any object from the VBA toolbox such as a label or checkbox. You may not have any macros but the msgbox still comes up when those objects are used.
Right click one sheet tab, select view code, under view select project explorer. Under that workbook you should have a folder called modules. open the modules folder and right click on any module in it, select remove module, when promoted to save select no, if you don't to save the module. That should do it.