Hi marsmimar and thank you for your reply.
In fact, I installed the Antivirus after a full format and Windows Installation. I did the scan one month later, found the viruses and cleaned them.
To tell you the truth, I've been using Avira free and Avast free for the last couple of years after using almost all existing antiviruses and I haven't really seen any difference between all antiviruses, but now, i am hesitating to continue using free security solutions since in my region (I live now in the Middle East) people tend to believe that paid (expensive) things, in this case antivirus solutions are much better.. therefore I am taking the decision to go for paid antiviruses, which takes me to the dilemma of which to use.
Here, people are more acquainted to Norton, Kaspersky and Nod32.. I am tending to go for Nod32 or kaspersky since the latter scan removable devices fully when plugged.