CIH (Chernobyl Virus) writes to the HDD MBR? not sure but writes to track 0 IIRC and to the BIOS but AFIK into the cmos portion..

There are some that will just corrupt your Cmos settings, some that that write to unused cmos areas (CIH), then there are the true baddies that flash portions of the BIOS firmware.. and I wish WinXP was immune.. but most of the victems I have encountered were Win9x and Directx 7.. these ppl also had their video cards BIOS flashed as well..ahhh phun and games..

BTW AT.. do a search on this board on BIOS.. there have been other discussions in the past on this matter..

Cheers