So you're saying that someone elses email address on your email client of choice correct?, is it a home system or at work? and does anyone else use the machine besides you?
Seeing someone elses email address on your system doesn't nessiarly mean someone hacked into your system(wouldn't make much sense for a hacker to use there email on the hacked machine and leave there username on it while there at it)
start with a malware and a anti virus scan. if your anti virus is old or outdated uninstall it. if your a comcast user they have nortons for comcast users. free anti virus is mse..avast..avg. install a newer version of your anti virus and do a full system scan.
if your system is clean, i would go into your email setup and change your email password. also if you use your password on facebook or other web pages change them. most times with emails...you get what called a blind send. where a spam bot will send an email to a group of people...you see 10-20 names on the top of the email...it look like you were sent or forward the email. some vendor now dont stop blind sends...some do. your better off if you dont know the people on the long send notice to delete the email.