There are at least two possibilities:
ONE:
Someone, at some past date, hacked into your account and copied your address book. It's not uncommon.
Those hacked details are now being used to send e-mails to your contacts, almost certainly not from your account.
There's nothing you can do about it directly. You could close down that AOL account and tell everyone to dump any messages from it.
TWO:
You have a virus on your PC which is sending out that stuff.
If one of the recipients of the spams is a bit pooter-savvy, he/she can look at the headers on the spam and see where it came from. That would clarify whether it's ONE or TWO above.