There are virus that can infect the BIOS and/or the boot sector.
For boot sector virus, you can simply fix it using Fixboot or fixmbr
or use a utility to fix it.
FixBoot is a small utility that overwrites a boot sector of a diskette with a standard boot code thus elimitating any diskette boot sector infection.
For BIOS virus, you will need to know what is the virus that infected the BIOS. In the above link there is also tools to remove a specific BIOS virus e.g. CIH virus, INT_CE boot virus
Note: Enabling virus protection on the BIOS help protect. In my BIOS, I always enable it after installing Windows.
Some anti-malware or anti-virus has boot sector scanner to check for Boot virus. Check with the documentation of the software if they have it.
Are these types of viruses common? What percent (a guesstimate will do) of risk do I have of acquiring one of these monsters, provided I use anti-virus software?
Yes. Depends on how it is spread. Thru infected floppy, email attachments or another network that is infected.
Just make sure your antivirus is always on and up-to-date and take advantage of the scanners ability to scan ALL areas.
(This is just a general question. I'm not having a problem with this topic, just curious.) I believe that it is correct that viruses can infect the BIOs, maybe I'm wrong. Is there special software to remove viruses from the BIOs? At one time I believe the BIOs chip could be replaced to solve this problem but now because of engineering changes this can't be done with modern mother boards. Is this correct? Are these types of viruses common? What percent (a guesstimate will do) of risk do I have of acquiring one of these monsters, provided I use anti-virus software?