Virus Protection
An anti-virus program is needed to protect your system from destructive software such as computer viruses, worms, and Trojan horses by removing them. This kind of software poses a danger to your system because it can erase important data or even cause it to crash.
Free antivirus programs are not as feature-rich as commercial products, but they do offer basic protection that can sometimes be extended if you are willing to pay. Generally, they are restricted to personal use. You can get this kind of free program from a vast variety of download sites like Download.com and Softpedia.
You can also buy them from companies like CliqueComputers. Most of these products offer an antispyware program and sometimes even a firewall program as well.
Fundamental information about viruses, Trojan horses, worms and antivirus programs can be found on Wikipedia.


Firewall
A firewall program, also called a "personal firewall", is needed to protect your system from unwanted attacks from the Internet and from software programs. Use of a firewall program is always advisable if your computer is connected to the Internet. It adds functionality to the firewall that is installed in your router in that it not only protects you from attacks ON your computer, but also attacks that can be unleashed FROM your computer onto the Internet. Such attacks can come, for example, from Trojan horses (see "Virus Protection") but also from software that you installed that sends your personal data to its manufacturer without authorization.
You can get a free firewall program, from virtually any major download site (e.g. Download.com and Softpedia)
Fundamental information about firewall programs and how they work can be found on Wikipedia.


Antispyware
An antispyware program is needed to protect your system from software that spies on your computer. For example, such software can be installed without your awareness while you are visiting a website and installing a different program from the same website. Most spyware programs observe such things as your browsing habits and the programs you tend to use and transfer that information to the spyware's developer. Use of an antispyware program is always advisable if your computer is connected to the Internet. Since spyware can also damage the functionality of your computer, it should always be removed. More and more antivirus programs and computer security programs on the market include an antispyware program or antispyware features.
We recommend that you use Windows Defender, included free of charge with the Windows operating system. Although some antispyware programs still have problems running under Windows Vista, you can still use such programs under Windows XP.
Find several free antispyware programs for download at sites like Download.com. You can get Windows Defender directly from Microsoft.