
I wont begin by trying to persuade you to purchase a particular antivirus package, or to use a free package such as AVG Free of Avast. Instead I will point you to a very reputable website that specializes in testing various security packages (Bottom of article).
I will, though, list my favourites and my recommendations:
If you insist on not having to pay for security software, then these are for you:
1. Avira Antivirus
2. Avast Free Edition

Thats it. I would only choose from the above two. Most people use AVG Free...but...well, I will reserve my opinion on that one.
Paid for security packages will almost always be better than free packages. This is simply because of funding. if you pay for an antivirus package then you are aiding the developers in research and therefore help them to keep there virus definition libraries up to date and accurate. i would recommend purchasing a package over a free one anyday. if you are looking to do so, here are my recommendations:
1. Eset Smart Security
This is Eset's complete and most secure security package, reason being that although it contains the base antivirus package, it also contain Anti-Spam and many other features for those who bank online. It has a far superior online protection engine than its sister Antivirus package.

2. Eset NOD32 Anti-virus
This is your standard virus protection package. It is not as fully featured as Smart Security but still contains the engine of its bigger brother. It is an extremely powerful anti-virus and is very light and friendly on your systems resources. it offers adequate protection from online threats but not so much for banking and larger online use. For that, look above for Smart Security, the fully fledged security package from Eset.

3. kaspersky Anti-virus/Internet Security
As with Eset, the difference between the Anti-virus package and Internet Security is the focus on online use for banking etc. Kaspersky is a great package BUT it is not as user friendly as the above mentioned suites. it offers watertight security but does require the use to train it in a nagging manner. nags aside, a very complete and well updated security package for slightly more advanced users.
4. Symantic Norton Anti-virus/Internet security
Norton has come a long way and is now lighter on system resources and gives a complete security package out of the box. Its a little pricey, but does a good job and so I think it is fair to put it in my list.
Now for that link: AV-Comparatives
These guys put all the packages available on the market, paid for and free, through their paces. Head over, and to the left will be a list of all previous and current performance, protection and removal tests. Check it out and make a decision based on your needs and what you have read rather than heard. A non biased source is always good and thats exactly what AV-Comparatives is.