Wow ... That is really weird!
1) try the uninstall in normal mode.
2) if #1 fails, try deleting the Sybase/PowerBuillder folder underneath the ProgramFiles(x86) folder.
3) run uninstall again. It should not find PB and thus cleanup the old uninstall registry entries.
4) reboot you PC
5) try the PB 12.6 install again. However this time, do a full install.
Good luck!