I dealt with this same frustrating thing for a week. Tried EVERYTHING listed here and finally got it fixed tonight!! Seems like various things have worked for some, but I'll post mine in hopes that it might help you ..
Workaround to Sudden Loss of Internet Access Problem
Overview : Microsoft Update KB951748 is known to cause loss of internet access for ZoneAlarm users on Windows XP/2000. Windows Vista users are not affected.
Impact : Sudden loss of internet access
Platforms Affected : ZoneAlarm Free, ZoneAlarm Pro, ZoneAlarm AntiVirus, ZoneAlarm Anti-Spyware, and ZoneAlarm Security Suite
Recommended Actions -
Uninstall the hotfix
1. Click the "Start Menu"
2. Click "Control Panel", or click "Settings" then "Control Panel"
3. Click on "Add or Remove Programs"
4. On the top of the add/remove programs dialog box, you should see a checkbox that says "show updates". Select this checkbox
5. Scroll down until you see "Security update for Windows (KB951748)"
6. Click "Remove" to uninstall the hotfix