System shuts down during startup - NIS 2009

I recently loaded NIS 2009 on my laptop without any difficulties.  I followed the installation instructions to uninstall my Symantec AntiVirus, but bypassed the ZoneAlarm uninstall, choosing instead to shut down ZoneAlarm.  (I don't have the install disk for ZoneAlarm anymore, and didn't want to lose my firewall entirely if Norton didn't work.)  The system ran fine for a few days, I ran a virus scan, etc.  Then I decided to restart my computer (which I only do every few days).

 

When Windows begins startup, the Start button appears, my desktop icons load, then at the point that the security program would normally start, the system hangs for a few minutes, then shuts down the computer.  (No error message, the screen just goes black like I removed the power source).

 

I'd forgotten to take ZoneAlarm off the list of programs that automatically load at startup (a bonehead move on my part), but after about a dozen restarts and some swift clicking, I managed to uncheck ZoneAlarm on the System Configuration screen.  I was even prompted to restart my computer for the changes to take effect, so I thought I was in the clear.  The system shut down (like it was supposed to, not abruptly like it had been), but when I restarted I continue to have the same problem.

 

I haven't been able to get back into System Configuration since then to check whether ZoneAlarm has remained in the startup sequence or not.  I only have about 5-10 seconds (with slow system response time) to get into any menus before it freezes, and on some restarts, I'm don't even get that.  I've tried Safe mode, Last Known Good Configuration, and Restore Default Settings (the only way I was able to get a System Configuration prompt before the freeze).  The only setting that seems to work is Safe Mode Command Prompt, but I don't know what to do once I'm there.  At this point, I'm wondering if it may be a conflict between Norton and my system rather than Norton and ZoneAlarm.  Is there a way to disable Norton from Command Prompt?  Any other suggestions?  It's an older system (second-hand, actually), but it worked perfectly until I installed Norton.  My husband had already put Norton on our desktop, which is even older, without any problems.  My OS is Windows XP.