I prefer Avast by far. I have been using it since about 2010 or 2011. I previously used AVG, and the free McAfee which was included with AOL dial-up. There are paid and free versions available. I've have installed this on many of my friends' and family's XP machines and no attacks even with the end of support 6 months ago. Excellent malware threat detection while loading malicious webpages. Also includes the option to install Google Chrome which is a good browser for XP now that Internet Explorer is no longer updated.
Runs well on older machines with just 512MB-1GB RAM.
Current version (9.x) => XP and later
Avast 4.8-8.0 => Windows 2000 and later
Avast 4.8=> Windows 95 and later. Updates are still released for all versions 4.8 and later http://www.avast.com/en-us/download-update