Someone hijacked my mom's email account HELP!?!?!?!?

Today my mom got all these "What the hell?" messages from her friends. It is found that someone hijacked my mom's email account and used her email address book to send all of her friends this program, and if you open it, you get a virus. Then we got this email from MAILER-DAEMON@excite.co.jp. My mom thinks that's the hijacker. What/who is exactly MAILER-DAEMON and what should we do?????