I am looking into antivirus programs, and three that I like the look of are Avast, MSE, and Avira. All three offer a real-time shield/protection/whatever you want to call it. However, I am worried that their shields might not be compatible with each other. What's the deal here? If they aren't compatible, which should I use? Thanks for the advice.
You don't want to run them together. You should pick one. They are all free, so you can see which one you like the best, and go with it. They all offer good protection. Personally, my top two recommendations are Avast and Microsoft Security Essentials. They both offer great protection. Just try both and see which you like the best.