There is a file in your user home directory ( C:\Users\Jentron\ac3dprefs8.txt on my machine ) that you can edit. Find the line that says:
Code:
window_geom_hier "462x458+355+177"
and take the numbers out so it reads:
Code:
window_geom_hier ""
Do this while AC3D is not running.
Geek note:
The third and fourth numbers are the position the window opens at, the first two are the window size.