BIOS can be flashed by viruses such as Magistr and some more.
Only way is to write protect it through some method (and update ur AV).
Visit the below site for more info about a product call BIOS Protect Easy software/hardware package which consists of
a write-protect jumper switch on the mainboard,
"Flash Write Protect" a built-in soft switch in BIOS,
and PC Cillin, anti-virus software.
*www.qdigrp.com/qdisite/eng/Inovation/easy_prot.htm