The best form of a computer security guard is a firewall. A firewall is a way of hiding your pc from the outside world (the internet).  Your computer is made anonymous and invisible to the internet while still allowing you access it.

Reasons for adding firewalls are to keep out hackers (especially important when you have a DSL or broadband connection),  and remaining anonymous which is necessary for privacy as more and more advertisers like to track users and their net habits. 

Firewalls can be built with software or hardware.  Hardware is better and more expensive.  Software is also effective if it is chosen and set up correctly. 

estimated price for software firewall:  $35

estimated price for hardware firewall (usually included in a router):  $75