After saving my own custom user interface and ui settings, Zbrush kept crashing on me whenever I would start up.
It would start fine, then if I so much as even touched any of the buttons or open file dialog, it would immediately crash.
As a test I put a tild mark in front of the CustomUserInterface.cfg file in the main Zbrush3 directory like this ~CustomUserInterface.
After doing that it seemed to startup fine and then I resaved all of my settings. So far, so good.
So if anyone else is having problems like this you might give it a shot.
If it dosen’t work you can always just delete the tild character.