1. When the system is powered off, some persistent BIOS settings are stored in a small bit of battery sustained RAM built using CMOS technology. The name "CMOS settings" have become synonymous with "BIOS settings". Some viruses have been known to corrupt these settings, resulting in a condition where the machine can no longer boot. Simply setting a jumper to disconnect the battery backup will restore the settings back to factory defaults. From Hacking-Lexicon