During my upgrade to the current version of ZoneAlarm free, my system became unresponsive on the reboot.
The welcome screen remained for a longer duration than normal.
My taskbar icons did not appear, the curser maintained an hourglass while overing over the taskbar or start button.
I could ctrl-alt-delete to obtain the taskmanager but there were no applications running (although the process list was full).
I could not gain access into any applications, or documents through normal methods (icons or the start button).
Since none of my other installs or upgrades ever took more than 10mins, I forced a reboot after 25mins, to the same issue.
Rebooting into safe mode was doable, but I was unable to uninstall the upgrade while in safe mode (and reinstall the earlier version).
If I remember from a previous experience, deleting the directory while in safe mode will not allow an earlier install (as the registry components did not uninstall, and the installer checks for a later version and requires the later version to uninstall prior to installing an earlier version).