PCs had bootblock viruses, only they could transmit from harddisks (only way to clear that was to clear the HDs partition table), I had a few of those (ParityBoot.B was a PITA, it behaved like the machine was overheating).
They could transmit if you left a disk (bootable or not) in the drive when starting, or by accessing it in some other ways.
Recently I've had to clear Sasser, Agobot (lots of those), Netsky, RBot, and a couple of other more oddball ones - these spead like wildfire through a WAN, and have caused me lots of trouble (we had Agobot several days before Windows had a patch to prevent re-infection - that was interesting!).
