How do I report to Yahoo spam being sent from my email account without my permission?

It's been happening for weeks now, my friends are getting emails advertising a canadian private health clinic! We are Uk and not interested!How do I report to Yahoo spam being sent from my email account without my permission?
They won't do anything. Your address book has been stolen. Make a new account.How do I report to Yahoo spam being sent from my email account without my permission?
This has happened to my hotmail account in the past. Change your password and will most likely stop. I noticed it because I kept getting MAIL DELIVERY failure messages, as a lot of people in my address book had changed their email address. After my password change, I stopped getting these emails.
http://download.cnet.com/windows/securit
Sounds like your E-mail account has been compromised, or you picked up a virus, Nicola.

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.