I have 3.1 installed in a separate directory.
Working WinXP Pro system with 100 gigs of disk free:
First try, Zbrush 3.5 tried to start (hour glass) fora full minute, then gave message that I did not have enough free disk space; then I clicked passed that and got opening zbrush window. I quit zbrush and tried again:
This time, and on the next two tries Zbruhs 3.5 failed to start entirely, gave no error message; hour glass just keeps running for a few minutes until I finally kill zbrush process with ctrl-alt-del
Have not been able to start it.
It sounds like something has caused ZBrush to have insufficient permissions. It must be able to write to its own folder and subfolders.
Also, have you tried disabling DEP for ZBrush?
–aurick