Yahoo IM security and i.p addresses in email headers

Received an IM from my defunct yahoo account - however, the password hadn't been changed, and I doubt anyone knows the password - even with a brute force password cracker, it's almost gibberish. Is using someone's account without guessing their secret answer or knowing their password possible?

Secondly, I've heard that you can find out an email sender's IP address from the email header. How do I do this?

EDIT: Would I, or Yahoo, be able to find the IP address of whoever sent me this IM (I have the log file)?

hmmm no i think u wud have to know pass or the secret questions cause it wud be pretty useless uf u cud jus walk in thru a bak door