Someone is sending e-mails from friend's e-mail address

[leoliver]

Hi Everyone,

I have a friend who called and told me that a unknown person has been sending e-mails , from his Yahoo e-mail address .

I immediately told him to download , install and run a full system scan with Malwarebytes. At the time I'm typing this thread , he just started the full system scan and already Malwarebytes has found 16 infected files.

I also told him to change his Yahoo mail password. Also, he told me that he had turned his firewall off , to try and speed up some P2P downloads , and forgot to turn it back on after the downloads had finished. His firewall had been off for several weeks before he discovered his firewall was still disabled. And it was at this time he found out that his e-mail address was compromised. He also has Windows Live mail on his computer , with a different e-mail address and no fraudulent e-mails were sent with that e-mail address , just his Yahoo e-mail address.

I have never dealt with this issue before , so I'm now asking for advice on what sort of malware could cause this problem.

And how a attacker could steal his e-mail address.

My Friend has McAfee Anti-virus on his computer , which detected nothing during a anti-virus scan. Is operating system is Windows Vista.

Thanks.