You first ought to do some research online to see the ratings and general abilities of each A/V or Internet Security product. Once you make a choice, use the web to find a local vendor who carries that product. Or, if the vendor allows it, you can buy a "soft" license from them and then download the A/V product. The serious vendors will assure that they have methods built in to their downloaded "installer" program to verify integrity of the download and to prevent its installation if the download files were corrupted.
As to the OTHER way to interpret your question: You've gotten recommendations for Norton and McAfee. I personally like Kaspersky. However, all three of those products have the ability to do automatic / scheduled self-updates. One of the most important factors in an A/V package is that if you don't update it regularly, you might as well not have it. You should also allow your 'puter to update its operating system regularly as well. The idea is that when you update your A/V and O/S, you become a "moving target" and the average-joe-hacker types will look for those dummies who DON'T update their systems. They will avoid you because you are more likely to catch them and interfere with their nefarious little small-minded schemes.