This may be a bit overboard, but you mind as well read a little about DOS attacks. DOS Whitepaper.

hatebreed is right - you don't have to have a static IP to be victim of a DOS, or even used in one for that matter. Some DDOS attacks are initiated via IRC. With that in mind, someone may have infected the victim with an IRC trojan that automatically joins a certain channel whenever connected to the Internet. From there, the attacker can issue commands to execute script/code to perform an attack. I should know; I was infected...haha