Viruses, Worms, Trojans, Spyware...
You heard them all, all the bad stuff you can get just from surfing on the internet. Your computer may be infected with viruses, spyware, and so on, and you don't even know it. Some are worse than others, one may make your computer malfunction, while others invade your privacy by watching your move on internet or sending you advertisements you don't want. You need to protect yourself, and your computer! And you can do it free, by simply downloading a few programs.
First off, the free software. I recommend using Avast 4 Home Edition: http://www.download.com/Avast-Home-Edition/3000-2239_4-10353376.html?tag=lst-0-1. It scans for viruses, worms, and trojans. It's one of the most popular free antivirus software out there, and has a good reputation of being reliable and catch all those viruses and what not. Of course, to be effective, the software must be updated every now and then with new information about new viruses. Updating is easy, either leave Automatic Updates on, or be sure to check up on new updates yourself.
If you want to buy one an antivirus software, go with Norton Antivirus or McAfee VirusScan, but the free Avast 4 Home Edition should be good enough for the average computer user.
Spyware can be very annoying, it can slow down your computer and send you unwanted ads. To remove spyware, use Ad-aware: http://www.download.com/Ad-Aware-SE-Personal-Edition/3000-8022_4-10319876.html?tag=lst-0-1 and Spyware Doctor: http://www.download.com/Spyware-Doctor/3000-8022-10361503.html?tag=list. Both are free programs. Even though both programs are free, they are the best spyware removal programs available. Using both of those spyware removal programs, you'll be sure to get rid of those spyware off your computer. Run both programs about once a week or two weeks (more or less, depends on you).
Ad-aware combined with Spyware Doctor will remove most, if not all spyware related files on your computer. In addition, use Spyware Blaster: http://www.download.com/SpywareBlaster/3000-8022_4-10305680.html?tag=lst-0-2. Spyware Blaster will prevent many types of spyware getting on your computer in the first place.
Now that you have everything that removes all those things, start to learn to surf the internet more safely without getting the viruses and spyware. There's many things you can do to minimize your chances of getting them, for emails: don't click on URLs on emails from people you don't know, don't download any attachments from people you don't know. For websites: if you visit a website and a pop up box asks you "Do you want to install and run...," click NO (unless it's something you know is suppose to happen and you know what it is). Just those few precautions, can save you a lot of trouble. If anything goes wrong, you have the the antivirus program and the spyware removal programs to help you out. But of course, it's better not to get them in the first place.