It is very likely because you are using windowed mode.
I have made countless UIs myself, and i have experienced this alot.
When using windowed mode you cut of some of the width and length of the screen in order to make space for your processbar in windows and the window frame.
Rather than 1920x1080, you might be using 1900x900 something, i don't know.. but it's kinda messed up at times.
Try running the game in fullscreen mode, or at least windows fullscreen mode at least, that might fix your problem as i removes all your windows frames and processbars and whatenot.
If that does not help, i don't know.
Anyhow, considering you have a 512mb video card, you might want to upgrade it for at least 1gb, or maybe more.
EDIT: Other than this, try deleting your WTF, Interface and font folder alltogether, and re-install the Interface you want to use, this solves a great deal of addon related issues.
I hope this helps, other than this i have no solutions