Microsoft Update KB951748 is known to cause loss of Internet access for ZoneAlarm users!
I suggest either Option 1 or Option 2 in the link below:
http://download.zonealarm.com/bin/free/pressReleases/2008/LossOfInternetAccessIssue.html
http://forum.zonelabs.org/zonelabs/board/message?board.id=cfg&message.id=52727
PP :)