1. Use an Antivirus

In the era of high-speed internet and portable media, you just cannot afford to compromise your PC and valuable data. So, Antivirus software is a must for each and every PC user. The Antivirus stays active in the system memory and monitors your file transfers, network/internet connections, and programs for suspicious activities and keeps away harmful computer viruses, worms and trojans from playing devil on your PC. It can even clean, heal or quarantine affected files already on your PC and keeps your data safe.

There are many AntiVirus Packages available, out of which many are free to use.
Some of the free packages are:

a. AVG Free Edition: http://www.free.grisoft.com
b. Avast! Home Edition: http://www.avast.com
c. Antivir Personal Edition: http://www.free-av.com

2. Use a Firewall

To protect your PC with network/internet connection from the prying eyes of hackers and abusers, it's a necessity to install a Firewall software. A firewall is simply a piece of software that sits between your PC and the outside world (network/www) and allows certain programs/data to pass through it, thus filtering the traffic based on configured rules.

Like Antivirus softwares, the list of firewall softwares is also long...and there are a number of free options available.
Some of the free packages are:

a. ZoneAlarm Free : http://www.zonelabs.com
b. Kerio Personal : http://www.kerio.com
c. Outpost Free : http://www.agnitum.com
d. Sygate Personal : http://www.sygate.com

3. Use an AntiSpyware/Adware

Every passing day gives birth to a score of new malware, which is any sort of program written solely to get itself installed on the victim's PC without his knowledge, monitor his activities/logins and send them to a hacker for identity theft. Similarly there are adwares which deliver unwanted advertisements and spoofs/redirects browser addresses. Thus, you never know that the form which you have filled 10 minutes ago is not from the original site ! To protect your PC against these nasty pests, you need an Anti-Spyware/Adware package.

Some of the free packages are:

a. Spybot S&D : http://www.safer-networking.org
b. Ad-aware : http://www.lavasoft.com
c. Windows Defender : http://www.microsoft.com/athome/security/spyware/software/default.mspx

4. Backup Often

It is really important to backup your valuable data often so that you can get a copy at hand when there is a system crash, virus attack or hacking. Just keep only the important files and folders as backup and not your whole disk! A habit of backing up never upsets you in the long run. It is a good idea to backup your data on a different hard-disk or removable, cheap media like CD, DVD, and Pen-drives to name a few. There are many free and paid softwares to choose from.

5. Follow Some Good Habits

a. Do not ever give out your password/login information to anyone.
b. Don't give out your credit card number or any online banking login to anyone.
c. Use a strong password which is difficult to guess; containing alphabets, numbers and special characters.
d. Password protect or encrypt confidential, valuable information to avoid being abused by someone.
e. Always set a password for your Operating System/BIOS logon.
f. Don't open suspicious email attachments or knowingly send someone an infected file.
g. Do not mount a known virus affected media to a PC without Antivirus protection.
h. Don't reply to emails asking for confidential login information even if they seem to come from the company or website. It might be a phishing attempt.
i. Do not reply to spam messages or scam emails like "You have won $10million lottery sum!". If you reply to spam emails, the spammers become aware of your activity and spam you even more. Just ignore them and keep the spam filter on.
j. Don't post your email id all over the internet (e.g.-forums)...spam bots might be harvesting them. Use myid[at]provider.com format.

To conclude, I would like to say that the steps that have been outlined above are not too difficult or time consuming to follow. You would certainly benefit by using those precautions and measures to secure your computer. Remember, if you secure your own PC, you not only help yourself, but many other users too. So spread the word and start acting now...Good Luck!