Someone is using my email account to send out spam. How did this happen and how can I stop it now and future?

I received this spam email sent from my account to my account (selling viagra, no less). At the bottom of the ad page was my email address again, looking as though it was the originator of the ad. "Contact us" did not work, and what the heck can I do?Someone is using my email account to send out spam. How did this happen and how can I stop it now and future?
I think many spam things do that to make you open it. But what I would say is to change your password maybe they got it from someplace? Or you might want to send it to spam. but your best bet is to change your password. Hope it works!Someone is using my email account to send out spam. How did this happen and how can I stop it now and future?
Sounds like your E-mail account has been compromised, or you picked up a virus, Alan.

Immediately have your computer checked for a 'bug' or 'bugs.' If you have the proper programs, do the scans yourself, if not you need to take your computer to some one who can.

Once it is clean of pests, install a firewall and an anti-virus program if you do not have one yet.

I strongly recommend a program for malware and adware also.

After you finish this, then go in and change your password, using a good strong one.

Also, check your account information and make sure no information was changed, if it was change it back immediately.

Then, you will have to inform your contacts that it was not you sending that mail, but they need to check their computers in case they were also infected.