Nearly every modern computer has "flash programmable" bios, that allows you to download updates and using some software, update the system bios. There are a few really nasty virus infections out there that are already capable of doing this. All you have to do is corrupt the bios, and your computer is now a beige (or black) paperweight. The only way to fix it (if you're lucky) is to return the motherboard to the manufacturer.
It would also be easy enough for a small assembly language program to delete the partition table, MBR, and boot sector of your hard drive, with the results as above.
Mark