By nature, you try to secure anything you find valuable or will encase items of value. In present age, (realize it or not) your information is one of the most valuable asset. If you use your computer for storing / accessing anything valuable to you then it is worth securing.
Even if you don't think the information on your computer is important, be a responsible computer user - by not maintaining basic security on your PC, you are most likely allowing your computer to act as a zombie, controlled by hackers to perform large-scale attacks on other computers (by background processes you are not even aware of).
Good News! To implement an essential level of computer security, you ( as a non-commercial PC user)  do not need to pay any additional bucks out of your pocket. There are lots of free resources on the web. In this article, I will be discussing five most essential but FREE tips. You can use these resources freely as per their terms of use as long as you want.

AntiVirus

It is absolutely must to run a effective anti-virus program, with updated definition files.  There are plenty of free anti virus programs out there, mine recommendation is AVG Free. In AVG free you get  a very professional antivirus software absolutely free; however, if you want a comprehensive security suite  from AVG; with integrated firewall functionality you have the option of commercial version. The effective workaround is use AVG Free firewall and get an effective standalone firewall (as discussed in next section).

Firewall

Although Windows XP Service Pack 2 or 3 and Vista include a basic firewall, they only monitor incoming traffic and will not alert you if some internal process like a spyware or  Trojan tries to contact the Internet. There are number of free firewall software offered on web. However, my recommendations are Comodo Firewall or ZoneAlarm Firewall. These are two of the most tested and award winning free firewalls.
Again catch here is, a professional firewall software absolutely free; however, if you want  to opt for a comprehensive security suite from these two vendors; with integrated antivirus functionality you have the option of their commercial version. The workaround is use AVG Free antivirus along with Free Comodo Firewall Pro or ZoneAlarm firewall. This way you get best of both worlds, free antivirus in form of AVG Free and free firewall in form of Comodo Firewall or ZoneAlarm Firewall.

Update Windows/OS and Browser

Keep your operating system (e.g. Windows XP/Vista, Mac OS X) updated with all the latest security patches for known vulnerabilities. For Microsoft Windows users, go to windowsupdate.microsoft.com. You can also allow settings to automatically update your computer, or ask/inform you of updates. Also keep your web browser software (i.e. Internet Explorer, Mozilla Fire Fox, Chrome) up to date with latest patches and updates.
Also tune your online security by changing the built-in security and privacy settings in your operating system or browser. Check the "Tools" or "Options" menus to learn how to adjust from the default settings. Use application specific "Help" functions for more information about detail security options.
Microsoft Windows comes with preinstalled web browser “Internet Explorer”, which has a history of security vulnerabilities. It is recommended to use a more secure browser like Mozilla FireFox (again free of cost)  with a better history for vulnerability and threat resistance.

Secure your Wireless Network

If you connect to internet, home or office network through a WiFi or wireless router then you need to cater for wireless security as well. There are plenty of resources for WiFi security issues; I am listing some essential steps in order of priority to secure your wireless network:

• Must change the default Administrator usernames and passwords for your wireless device.
• Turn on WPA / WEP Encryption compatible to your wireless device and PC/Laptop/Handheld.
• Change the default SSID of your wireless device.
• Disable SSID broadcast on your wireless device.
• Turn Off the wireless network during extended periods of non-use.

Backup

Following the above four tips, you're more likely to be free of interference from hackers, viruses, and spammers. But no system is completely secure. If you have important files stored on your computer, copy them onto a removable disc or an external hard drive, and store it in a safe place. Alternatively you can use a free backup utility like Comodo Backup.