Hi, Gwynant
It sounds like your Yahoo account has been hacked.
If you access your mail online using your browser (internet Explorer, etc) Normally, just changing your Yahoo password will sort this out.
But if you are using an email client, eg. Windows Mail, You should scan your computer with a program like Mbam, Superantispyware or S&D as themedz recommended.
These programs are free & scan for & remove most malware. - Personally, I use Mbam (malwarebytes anti malware). If you use this program, when you install it, untick/tick the trial box so that you are NOT using the trial version.
The emails that have been sent will prob only be advertisments, so any recipients should not have any problems with them.
Saying this though, you should send them an email to warn them not to click on any links contained in them as a precaution.
Rob