Sadly, a free Antivirus protection won't cut it in Windows XP. Keep in mind that as of now, Windows XP's support has been discontinued, which means that you are using an outdated and vulnerable OS, no matter how you look at it. Even if you install a security software for every aspect of the OS, you won't be able to protect it from being exploited at a system level. It's strongly recommended to either upgrade to a newer version of Windows (at least Windows 7) if you want to use a free Antivirus solution, or move to a paid Antivirus program for Windows XP as like I said, a free Antivirus just won't do it.
On top of that, is it possible to have your computer specs and know what you use that computer for, so we can give you recommendations based on your needs? Keep in mind that most recommendations here given by the members are based on their opinions and experiences with the products, since the "best" Antivirus program doesn't exist and probably never will. In this domain, the "best" is relative to a person's point of view, and isn't established by an organization or anything else.