Email hacked?

Since I changed over to outlook.com several months ago, I have had multiple instances where I guess my email either has a virus or is being hacked.  From my email, someone is sending out emails to people who used to be on my contact list (they are not on my contacts anymore).  The only reason I know is because emails that no longer exist get kicked back to me and I get Mail delivery failed. I have changed my password, installed Microsoft essentials, Norton . . . it stops for a while but then comes back a month or so later.  I am ready to just delete the account but then will I lose all of my contacts?  Any help is appreciated!