Computer Viruses to be exact. Let's discuss the difference between Computer Viruses and the Flu.
You can try and prevent the flu through doing things like washing your hands, staying away from public areas, and not putting things near your face that haven't been disinfected. You kind of have to work to "not get sick".
Computer Viruses are named badly. They aren't like viruses at all except they make your computer sick. With Computer Viruses you almost have to work "to get sick". Computer Viruses would be more appropriately named Computer Hangovers and here's why:
They are caused by you trying to install one piece of free software too many. My daughter gets computer hangovers all the time because she likes trying to watch "free" movies before they are released. You go do an Internet search for "watch avatar 3d for free" and you'll get lots of offers. 9 out of 10 of them probably have a hangover attached. The other 1 just installs spyware which is very similar just not quite as malicious.
Of course, companies like Symantec and McAfee don't want you knowing this because their business model exists based on public fear. I've never used anti-virus software and I've never had a computer virus. You just have to follow one simple rule: if you don't trust it 100%, don't download it, don't run it, don't open it, and definitely don't install it.
Windows Vista and Windows 7 have vastly approved upon this rule. Everytime something tries to change your system Vista will open up a dialog asking you if you it's okay. You can go one step further and make all "user" accounts unable to say it's okay without a password. This way if you have children who are trying to get free movies, when the virus tries to install itself, they'll get the "is it okay" dialog and it'll ask for a password which you, as the parent, have intelligently not shared.
Normally, once you are infected by spyware or a virus, it's not worth the time and trouble to fix it. You'll be 50/50 at best and most of the software is good enough that the act of removing it will debilitate your computer. You'll end up spending 8 hours trying to remove it, 4 hours making sure it's completely gone and the next 2 hours crying about wasting 12 hours.
I'm not good at removing viruses. If your computer has become debilitated from a nasty hangover, you can sometimes revert to "the last known good configuration" and this will sometimes get you back up and running. If it doesn't, you will probably have to find your installation CD/DVD (which most people I've found have no idea that such a thing exists) and reinstall. If you realize that Win XP was installed with a disc and you have access to it, you should follow these instructions...
To get to the "last known good configuration" screen, just start hitting F8 as soon as you power your computer on. It actually is a boot menu just after the BIOS screen flashes by, but I find it's safer just to tell people to start tapping the F8 key as soon as they turn on their computer. You should get to a boot menu that looks something like this: