Install a two-way firewall
While anti-virus software scans emails and files for viruses, a firewall blocks access to and from uninvited sources. If you already have a firewall installed on your computer, make sure it is turned on.
Some firewalls are only 'one-way', that is, they only block hackers trying to access your computer from the Internet. A true firewall will block both incoming communication as well as anything being sent out of your computer without your authorization. If you do not have a firewall or if your firewall is only one-way, consider purchasing a new one.