One of the latest Windows Update KB951748 when installed will result in an instant loss of Internet access.
I know. I updated my home computer this morning before leaving to work. Came home tonight, my business computer worked fine. My home computer wouldn't connect. After trying different ports on my hub, I tried a new Cat 5 ethernet cable...Contacted my cable provider.
You can set your Internet Security to Medium and go to the Zonealarm home page....technical support.
There is an update date 7/9/2008 with a temporary workaround.
It only applies to XP and ZoneAlarm. Users of Vista are not seeing the problem.
Good luck.