ZoneAlarm is a type of firewall called a "personal firewall". That is, it's
a piece of software which runs on the PC that it's protecting. 

ZoneAlarm has a feature which intercepts all VNC keyboard and mouse inputs 
whenever a ZA window is active.  This can be caused by an alert box popping 
up on the VNC Server, or by a user on the VNC Server activating the ZA 
control panel.  Once this happens, VNC behaves as if it is in 'view only' 
mode -- the user on the VNC Viewer can see the windows on the VNC Server, 
but can no longer control them.  

If and when someone closes the ZA windows on the VNC Server machine, control
will be returned to the VNC Viewer.  Until then, no amount of reconnects 
will regain remote control of the server.

This 'feature' of ZoneAlamr can be turned off in ZoneAlarm as follows:

- Open ZA control window
- Click the 'Firewall' tab
- Click the 'Advanced' button near the bottom of the tab window
- In the 'Advanced Settings' window, turn on the option titled 'Disable 
  Keyboard and Mouse Protection'.
- Click 'OK'.

ZoneAlarm has other advice for VNC users here:

http://www.zonelabs.com/store/content/support/zapProgramsFAQ.jsp