This week I’ve received 2 Spam messages from 2 different friends who use Yahoo Mail.
It looks like someone has been using their account to sent Spam to all contacts they have. Is it because their computer is infected by a Virus?

This is a  very common problem. Usually these hackers are able to gain access to email accounts (MSN/Hotmail, Gmail, Yahoo! Mail, yMail, etc.) without ever having to touch someone’s PC, so the odds of your friends’ computers being infected with a virus is low. It starts by receiving a suspicious email, there are all sorts of these. Usually curiosity causes people to click on the link within the email. Once you click on the link, you basically give the malware permission to access all your contacts, and then the spam begins. The best thing to do in this situation is to change your password AND security questions

That said, there IS a chance that their PCs (or at least one of them) are infected, so it is always good to have them checked over by a competent technician. If you think you may have been infected as well, have your PC looked at by a professional.