Firstly, never believe that an antivirus is perfect and provides total cover for your PC. New viruses are coded everyday and it all is about as fast paced as the IT sector itself. Take a few precautions to help you take care of your PC and data.
1. Don't go to sites that are virus carriers.
Easier said than done, because you can never tell when a site is going to drop a few unexpected packages on you or not. A lot of browsers now days have (Chrome does this more often than not) a system that's used to identify sites that will either attempt to infect you and will block that site immediately and will not allow you to enter it. Some antiviruses do that as well, with Trend Micro being the most finicky. Some antivirus software also have a rating system (avast, avg) and they will show you sites that are safe (green) neutral (grey), or malicious (red). So sticking to the green is always a good idea. Grey is not that bad, but if the browser or antivirus complains, get outta there

2. Don't open emails when you don't know who the sender is.
Happened to me once, and the little

3. External sources like CD's
A lot of the time, antivirus software gives you the option of scanning something on its own, by right clicking on the file and then choosing to scan it. This is often the best thing to do with a Cd you got from someone, and that includes discs that you get with magazines. You cannot say for sure how good someone else's knowledge about viruses is and whether or not they are protected from viruses.
A few things to look out for:
A) Your system slows down to a crawl, and you have a more processes running in your task manager than there should be. This is easy to see, you just press control alt and delete and it brings up the task manager. When you install new software, just take a note of any extra processes (or screenshot them). When you suspect that you may have a virus, go take a look at the screenshot and compare the two.
B) Your browser opens pages that you have never visited before in your life. it happens. Many browsers actually save the last tabs left open from your last session, which you can disable in the settings of said browser.
C) You get a notification on your computer saying your PC is infected, and proceeds to throw a ton of adverts at you for antivirus software. That's an old one though, but it was one of my funniest.
A virus can be used for a lot of different things, and the most likely is to either turn your computer into another drone where they can then infect more computers, or they can use it to infiltrate your data and take anything they want really, so its important not to keep passwords, banking data, or anything else that is sensitive on a PC.
If you need antivirus protection, go with the names you can trust, that have been around for a long time. Many times an antivirus is written and it looks like it works perfectly, but then its just a shell that is written to protect you from some, and not others (like the ones coded by the same people that created the antivirus).