The Virus Factor

Malware, a virus and other annoying pieces of code like spyware or worms are a death to many people when working on computers. Since the beginning of computers there has been a birth of hackers and "crackers" that program and design a virus for various purposes. To effectively deal with this, there's a few things you should know about the nasty things.

A virus in itself is a self-replicating program that causes harm to a system. This usually include infecting files on a target computer with the purpose of damaging the computer, or compromising the system to enable a hacker easier access.
Malware has more specific uses such as damaging specific files to render a system inoperable. A virus or malware usually writes itself into multiple locations, and into the registry to make it harder to remove.

There is of course other types of malware and unwanted programs. Spyware is installed usually with the sole purpose of retrieving information from the infected machine such as applications that are being run, or keystrokes (to record passwords or banking details), etc. These are especially damaging to a person if their money is stolen through the use of spyware.
There is also trojans which activate on being run, or worms, which specialize in spreading through networks.

All these software leave a computer in tatters and can be very annoying to get rid of.

Preventing these are the best way to handle this. This can be achieved by having up to date and effective Anti-virus and Anti-malware applications installed on your computer.

Usually the only thing you'd need in my opinion is Microsoft Security Essentials. Designed and updated by Microsoft, this is the best anti-virus I have come across. It's small, light and very effective at preventing and removing virus threats from a computer. MSE even prevents most malware from getting a foothold on your pc, and I've recovered computers that were extremely unstable by using MSE to clean it up.
If you feel you need more protection, there is various anti-malware and spyware programs out there, of which many are free. On my old XP machine I used to use Spybot Search and Destroy. It was very strict in registry writes, and you have the choice to allow anything written to registry. This is kind of annoying after a while, and it isn't light on resources.

If your computer gets infected, and an anti-virus cannot remove it for you, then the best way to get rid of it is consulting with malware experts. Found on the internet everywhere, they will use programs such as HighjackThis (which you run on your computer to generate a log for them), to find and help you remove these hard to destroy malware that you may find on your computer. I can suggest the Security and Malware-removal section over at TechSupportForum if you need any help with this.

If you have any questions about malware and virusses feel free to post a comment, and I'll get back to you.