Someone has either
a) hacked your email account and sent a lot of spam mail from it or
b) has got hold of your email address and is sending spam mail from another machine but using your email address as the sender.
For a), change your email account's password to something unlikely to be guessed and check your sent folder to see if there are any messages in there that you did not send (if there are, the account was hacked).
For b) create another email address on Gmail, Hotmail.Live mail or Yahoo and tell your friends, family and other genuine contacts to use that one from now on. Do not use the new address to sign up for anything on line, and do not publicise it on line in any way. Then either delete your current email account and create another one you can use to sign up for things on line, or keep it but reserve it for signing up to things.
I have several email addresses - one is strictly for friends and family, one for work, one for signing up to sites I believe can be trusted, and one for sites where I'm not 100% sure they can be trusted. The first two are never publicised in any way..
The other advantage of using things like Gmail is that by not using the email address I get from my ISP, should I ever decide to change supplier, I don't have to worry about making sure all my contacts have the new address.