A virus is a program that replicates itself. It cannot exist on its own so it attaches to another program, usually an executable one. A worm is like a virus—it also replicates itself. However, it can stand on its own and does not need another program to run. It usually infects your computer's networking features, which includes its internet connection.

Spyware and adware go hand-in-hand. Spyware is a kind of program which gathers information, specifically browsing habits. It tracks down what kind of sites you visit, and the adware will generate ads that fit with your interest which the spyware based on information it gathered in the first place.

Malware is malicious software. It's any program that is useless, or worse, destructive. A Trojan a program pretending to be good but once it's deep in your system it proves to be quite the opposite.

In spite of all their differences, they have one thing in common: they are out to give you headaches by rendering your computer unusable.

To protect your computer as much as possible from being infected, here are four dangerous activities that you should avoid, or at least minimize:

• Opening email attachments

Do not open any email attachments if they aren't scanned by antivirus software. Worms can spread through email, so even a friend can unwittingly send you one by way of an attachment.

• Internet File-sharing

In file-sharing via the internet, your computer is exposed and open to others it is communicating with. If other computers' files can be transferred to yours, the same thing can be said about a virus, if the others are infected.

• Downloading free software of questionable origins

Free software is free for a reason. If you bothered to read the End User License Agreement (EULA) of a software you are installing—which you probably didn't—you most likely will come upon a short and tiny clause saying that if you agree to the terms, you are allowing advertisements to pop up on your screen, or other software to be installed in your system. So be careful with what you download. Read product reviews and find out if other people got headaches from using it.

• Visiting ad-heavy sites

If you visit a site and you're immediately bombarded with pop-up ads, leave immediately. You might click on an ad that activates an automatic download of malicious software to your computer.

Here is a list of activities which you should be doing instead:

• Invest in good antivirus software. Going online without the protection of antivirus software is like going out naked in the snow. That's just how vulnerable an unprotected computer is.

• Most antivirus software packages include an internet security program (also known as firewall). A firewall acts as a barricade between you and unwanted content from the internet. When buying antivirus software, it's better to shell out for the internet security as well.

• Once antivirus software is installed in your computer, keep it updated always so it can recognize and remove newly released viruses.

• Whatever browser you are using, check that its security settings are all in default unless you really know what you are doing.

• Periodically delete files that you no longer use.

• Download spyware- and adware-removal tools and run them at least once a week. Be warned that many of these removal programs are a scam, so make sure that what you download is reliable. You can easily search for product reviews to check if the program is legitimate or otherwise.

• Back up all your important files all the time.

• If possible, have two computers at home. One will be used to connect to the internet, and the other should not be connected at all. The second computer will act as storage of all your important files so that even if the first one crashes, your life won't be in ruins.

Don't be dependent on removal tools and antivirus software packages, though. A bit of prevention is always worth a megabyte of cure.