how do i get rid of this weird message that pre-exists when i compose an email??

i bought a new computer, and when I went to yahoo to write an email the following message was pre-existing in the little box where i would write the message…. So every time i want to email someone i have to make sure i delete before i send.

the message…

Hello,
Dear friend, How are you doing?
I find a site to sell electronic products with very good price. Laptop, DC and Cellphone even Motorcycle are very popular. Their products are original quality with very low price as wholesale business supplier. They also can do retail business for end user now. Maybe it is fit for your business . If you like you can contact them : <peofc.com >

Wish you everything goes well.
Best regards.

Address: No.57 Chengdongnan Road, Zhengzhou, Henan,China

Zip Code: 450000
TEL: +86-371-61725986
Email: peofc@188.com
MSN: peofc01@hotmail.com

Your account has been compromised, you need to change your password and do a full virus/malware scan.