I had the same problem, I suspect it being windows 7 service pack 1, I had this problem a week ago so I formatted my PC and FFXIV started working normally on full screen, and after installing SP1 it stopped working on full screen but works in windowed mode. I did a system restore and removed SP1 and it started working normally again on Full Screen.

However, I want you to try something. Open FFXIV properties (right click on FFXIV icon) and under "Compatibility" tab tick "Run this program in compatibility for mode:" and choose "Windows 7" and see if this solves your problem. As now i'm refraining myself from re-installing SP1 again. I might do it if Full Screen works for you again.