You're probably thinking of a boot sector virus...not as common as they used to be but are going through a resurgence as the newest batch of malicious idiots are cutting their chops in the malware world.
They are particularly hard to get rid of and usually need a floppy based AV to sniff them out and remove them or a complete low-level format and re-partitioning of a HDD in extreme cases.
No, not thinking of an MBR virus.
(Easy fix for those since I had a problem with a Windows 3.0 instalation disk at one point:
fdisk /mbr
re-writes the Master Boot Record. It can even clear a program like Lilo, so it can be problamatic)
After my research, (From GX1_Man's link to Google), I found out that they do exist, and that they can be quite dammaging.
I had an uncle who said at one time a long time ago that he had a BIOS virus, and I wanted to do more research on it some day. (now I know)
The BIOS are infected through the BIOS Updates (which, on many BIOS chips, can be dissabled to prevent such a virus)