This is ZoneAlarm's Press Release of the problem, and their recommended actions to resolve it;
This seems to be an increasing problem from today.
Updating Windows XP with the KB951748 update may disable your internet connection but only if if you also use ZoneAlarm as your firewall.
The quick solution is to carry out an immediate restore using System Restore. Windows will have created a restore point immediately before installing this update.
You can access System Restore by going to Start > All Prorgrams > Accessories > System Tools > System Restore. Unless you have created other restore points on the same day you installed this update, there should only be one restore date/time available. Make sure that is selected, then follow the prompts by clicking Next.
There are more details of the fault here;
And details of ZoneAlarms connection here;
Please use either of those discussions, or create your own new thread, if you continue to have problems or need other advice about this.
I hope that helps.