How can the risk of unauthorized computer system access be reduced ?

Apply authentication security like password, Install good antivirus program, always scan for Trojen Viruses which helps the hacker to hack the PC[ex: Trojen Horse- the best trojen and PUP finder BAIDU ANTIVIRUS], Enable firewall for the network if the computer is a part of network and moreover stop clicking on all the link that you receive by mail or message.