I am terribly short on time at the moment, but I just have to record this:
Don’t use the Zone Labs ZoneAlarm Firewall on Microsoft Windows Vista. Just don’t do it!

For me, the firewall caused numerous seemingly unrelated problems: the Windows Update panel froze when started (had to kill explorer.exe), the Windows Defender updates were not working anymore, I could not install Visual Studio (Visual C++ 2008 Express to be precise), etc.

I will certainly write more about this somewhen in the future…

Apparently, Microsoft have already covered this issue in a knowledge base entry almost two years ago. D’oh! I will probably do some testing on this during the next days.