In the last year, a series of viruses and worms that caused damage across the Internet in record time has made very clear how vulnerable computer systems are. The MS Blaster, Slammer, Sasser, and Korgo.W worms have shown that signature-based antivirus software and traditional firewalls are not enough to protect networks. Everyone is worried about a zero-hour attack--an attack based on a previously unknown vulnerability and completely immune to antivirus software. This article offers suggestions on what can be done to protect networks from such an event.