Its not your email account that has been hacked. Changing your password is not the answer. You most likely clicked on a link in a similar email and this sent you to a scam webpage which then put a virus onto your computer. This virus has activated itself, went through your address book and send the 'spam' emails to everyone there. It will do this periodically until it is removed. It has probably got itself well installed onto your PC. It is likely that it has hidden itself in the rootkit of your operating system. These trojan viruses (virii?) are very difficult to remove and most virus protection applications will not protect you from them. Getting rid of them is very difficult and best left to a computer professional to track them down and remove them using special s/w tools.