Criminals and other unethical folks have devised many ingenious ways to invade your computer and mess things up. Malicious software ("malware") can take many forms, from viruses that can seriously harm your PC, to pop-ups, browser hijackers, ad-ware, etc that can cause annoying slow-downs and even outright crashes on your machine.
It is very important that you practice caution when surfing the web, especially if you plan to download software, particularly the "free" stuff. Often free software comes bundled with additional programs that
cause problems on your computer. If you do decide to install the software anyway, watch the installation steps carefully, and make sure you opt out when asked if you want to install any program other than the one you downloaded. A good article on safe browsing can be found here.
Everyone should have a good antivirus program installed on their computer. Windows 7 & 8 have a built-in program called Windows Defender, but it is not considered as effective as some of the alternatives. Although you are free to pay for a program, there are many excellent free options available. Avast, Bitdefender, Panda, and AVG are just a few.
Generally, you do NOT want to have more than one anti-virus program installed on your computer at the same time. They can conflict with one another and cause system slowdowns.
However, in addition to your anti-virus program, you SHOULD have an anti-malware program installed, and run it on a regular basis to detect and clean malicious software files you have picked up. Personally, I like Malwarebytes. The free version, however, only detects and cleans AFTER the PC is already infected. The paid version offers real-time protection (prevention). There is another freebie, spywareblaster, which claims to actually prevent infection. I may give it a test drive and report the results in a future post.