A friend of mine has been spending the last three or four days repairing the damages that have been caused by an email virus that is on yahoo. Apparently there is an advertisement that has a virus attached to it. Once you access the page to GET your emails, the virus attacks your computer. Unfortunately there has been NO announcement about this as of yet. Check your emails at your own risk.
try installing security softwares that do scans on emails. and also like the 2nd post said, better use email clients (outlook, outlook express, etc.) to get rid of ads, and unwanted pictures and emails spams which normally carries viruses.