I doubt that a virus can easily get in/on your motherboard, the only potential for this if if the BIOS somehow was infected (it is hard enough to update your BIOS let alone infect it), difficult but theoretically possible. A google search for BIOS virus returns many, many hits -
http://www.google.co.uk/search?q=BIOS+virus Don't go frightening yourself unduly.
However, things like this tend to be HOAX/MYTHS and your first port of call should be
http://vmyths.com if you receive something like this in the mail.