how do I get rid of spam on yahoo.

I have changed my login. and everytime I send an email the people I send the email to gets an ad for male enhancements. I have crashed my computer new antivius software. and it is on yahoo. I have asked them about it and got no answer. and this morning I recieved a message that I was on two computers. so is some one else saying they are me using yahoo? and what are they sending? how do I stop this.

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