Working in the computer industry for over a dozen years and repairing ten's of thousands of computers over that time has given me rather good insight into some of the most common mistakes home users make. Almost half of my computer repair business is from clients who are infected with a virus. Removing a virus from a computer can be easy at times and other instances it seems like hell on earth.
The most common mistake end users make is not having real protection from viruses. Let's look at AVG for instance. Their security client is good and millions of people use them. However the users who have the free AVG client installed don't realize that the free version of the software does not block viruses up front. It can only detect them after you are infected with a virus. The same goes for many other free based clients. If you were in a war you would want a bullet proof vest and helmet for protection. Basically it's the same thing with anti-virus software. You want to be protected before the damage can take place. Getting shot thinking the doctor can just stitch you up later is not a good way to go. Waiting till after you are infected with a virus is similar, the damage is already done and no you are left with having to remove the threat and un-do any changes the virus may of caused.
Another free based client is Avast. This client does offer live up front protection. What most users of the free based version of Avast do not know is you do not have the latest security updates. Only the paid versions of the above software will have the latest updates that you need as well as the up front protection you require. In no way am I saying the above clients are not good. They are. However you need to be fully protected online with up front protection against viruses so you do not get infected in the first place and have the latest security updates from that client so that anti-virus software can block those new threats.
In my field even the pros disagree on what works best and if free clients are good to use or not. My personal stance is you get what you pay for. If you are not willing to purchase protection for your computer you are leaving yourself open to attack. 90% of all my clients who got infected from viruses were either using free anti-virus clients or no anti-virus software.
Even with the best paid protection out there you still need to be proactive. Anti-virus clients are not fool proof and from time to time a virus can slip in. Most of these cases can be avoided through safe internet browsing. Don't download software unless you fully trust the source. If your on a website that you have never been to before and it's asking you to install the latest Microsoft update of video codec you should not do it. If your on a social network and see a link to something funny you should not click that link unless you trust the source and fully understand were that link is taking you. Another tip is to not ignore security client warnings. If you have a client that is warning you about a certain website you should listen to it and not visit that site.
In the end only you can decide if you want your computer to be fully protected. Free based clients are great to use to scan your computer for threats but I would not recommend using them as the only line of defense.