In my bulk folder many messages are coming with extension *.mim,*.hqx,*.uu.?

when i open the mail it is alerted that it is virus affected. The messages are from my friends whose id's are in my address book. When i asked it to them they told me that they ddnt send such a mail. Now it is happening frequently. Is this cause any harm to my yahoo account or pc. How can i save mu account. Though yahoo server has in built protection of norton anti virus how can a intruder post a virus affected mail