WIN XP will never be as secure as either WIN 7 or 8. Also consider dumping IE8 as support apparently ends with WIN XP.
Browsers are have become the number one application patched for security vulnerabilities. Switch to a more secure, updated browser supported with updates such as Chrome or Firefox.
Consider replacing your current AV software with one highly rated by independent security organizations. Both
Bitdefender Antivirus Free Edition and
Comodo Free Internet Security are top rated for protection. Followed by
AVG AntiVirus Free 2014 for WIN XP.
Comodo is apparently more resource heavy than either Bitdefender(lightest) or AVG, and can be a little more confusing to use its built-in features, but does offer a basic sandboxing mode. This is designed to isolate your browsers, such as Comodos Dragon browser, or unknown applications from making critical system changes typical of malware.
I'd also keep Malwarebytes installed locally, and consider also using a keyboard encryption app such as
AntiLogger Free. Just make sure it runs automatically at startup, as running after the fact could render it useless.