Because the Internet is used more and more often as a business tool, it is vitally important to secure computers from outside interference, hacking, and viruses. While there are many good methods of securing a computer, the best is to not connect it to the Internet at all; a disconnected computer cannot download viruses, malware, or spyware.
Since this is impossible in most situations, the next best solution is a firewall program combined with a powerful anti-virus program. These are available both free, for home use, and specifically for business use. Windows includes a built-in firewall that is reasonably secure, and Microsoft Security Essentials is a free and reliable anti-virus program, updated regularly.
There are also many third-party firewalls and anti-virus programs specifically for business networks; McAfee and Norton are two popular and reliable programs for sale, with free trial versions. There are also website blocking programs that prohibit access to many dangerous or infectious websites; these are commonly used in corporate networks to diminish virus attacks and decrease casual browsing.
Firewalls which block incoming access are a good way to prevent Denial of Service attacks or outside hacking, and there are free programs such as PeerBlock for home use that block a computer's ISP from outside access. Google's search engine also has a reliable scanning system to identify potentially harmful websites.