Just know that there are good free programs out there, but there are things you should know about them.
For example, you may need to do with an occasional update or popup advertisement to use it, and it won't provide a full feature set. I know with Avira's Free edition, every time you successfully connect to update the product and definition files, you'll get a popup with current deals on the product. For most, this is a minor nuisance when you consider the free protection, hitting ENTER or the Spacebar key will remove it.
Another example - some free versions of antivirus may not offer scheduled virus scans, at one time this is something common of many free antivirus programs (Avira Free DOES allow scheduled scans - thanks, Cecil). This may not be a problem for most people, because the real-time scanning and pre-boot scanning options will hunt for viruses each time you access, read, or open a file, so it should technically catch any virus prior to it being able to run. You can always start scans manually.
You may also find that some free antivirus solutions do not offer additional components such as email-scanning, rootkit detection, or web browser security. All reputable vendors provide a typical 'feature grid' at their website to show you what is available in each version of their product. Always look for this list before you choose to install the product, so you know waht kind of coverage you have available.
Many lesser-known products can offer better overall protection at a lower rate. So, whereas McAfee may cost you $50 for their product, something like, say, Avast!, may be only $25.
You also need to consider "residual costs" - residual costs are the biggest ploy in the computer industry. You get people to buy your product at a low rate, and need to continue paying for subscriptions every year. For some products, like McAfee or Norton, you will find a yearly update surcharge. Other products for home use will NOT have a charge like this.
Biggest example of residual costs in the industry that I can think of: Printers. You know that ink-based printer at home? You know how expensive the cartridges are for it? Almost none of them are refillable, and it's hard to find generic ones that work. So, how do printer companies like HP and Canon make money? Charge $25 for the printer, knowing that a buyer would rather pay less NOW, and also knowing that the buyer will be purchasing $100/year in ink from you, and the printer will die in 2-3 years because you made it so cheaply, so they'll be forced to buy another from you! Realistically, you could have bought a toner-based printer for $100, and spend $50 every 2nd or 3rd year for toner, and the printer would last 8-10 years. Much less e-waste in the landfill also, since so very few people e-Recycle these days.
Oldest trick in the book ;)