Whatever Antivirus software you use, it can not completely elminate viruses, trojan and spywares. Once these make an entry into your system, then formating is the best way to be away from viruses.
If you are a home user, you can use Avast free of charge. Register with a free e-mail account. The company will supply a code with one year free subcription for updating the antivirus database. You can renew your code at the expiration date.