For securing your computer systems from viruses you should install a good anti-virus software on your computer system.
Anti-virus software will help protect your computer by maintaining a database of viruses and eliminating viruses and threats to your system.Anti-virus software can only effectively work if you keep it running at all times and update it frequently. For this purposes you need a internet connection.
You can also secure your computer from viruses by having a secure Internet connection and combining it with a firewall.
You can use either of the below listed Antiviruses program to secure your system:
1.Bitdefender Antivirus Plus 2013
2.Kaspersky Anti-Virus 2013
3.Avira Antivirus Premium 2013
4.ESET NOD32 Antivirus 6
5.AVG Antivirus Professional 2013