Spam being sent from my Yahoo mail when computer is turned off?

Sounds like you picked up a virus.

Immediately have your computer checked for a 'bug' or 'bugs.'

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.

Then you have to inform all of your contacts that you did not send them those mails, the virus did, and that they need to check their computers for infection, too

That problem is not caused by any type of virus or adware on your computer. At some point someone obtained your ID and password and took all your contacts. From that point on they do not need your account. Anyone with a separate email program can use any address as the FROM address. Spammers use your list and your name so people will read the spam. There is no way to stop it at this point. Change your password to keep it from happening again.

₪ ʎəɿʞɹɐq ₪

bad news.

at some time you have answered an e mail or something which has compromised the security of your account.

you probably have answered a phishing e mail which has allowed the spammer to get into your contact list.

it may be necessary to contact Yahoo to see if they will have you close the old account and begin a new one, or assist you in somehow removing the spammer's access.

best of luck.

I think it possible that your contact list was read by a virus in one of your contacts. Alert all your contacts that your address is being forged and that they may be infected.

And is your McAfee fully up to date?

Mike may be right. Also, ask this question under "Computers and internet"