Hi, sky. Welcome to LandzDown Forum.
Footprint-wise, I think Microsoft Security Essentials is probably your best selection. On the other hand, Avast scored well on the Virus Bulletin tests.
http://www.virusbtn.com/ I think you would be happy with either as long as you have a software firewall in place, all security updates installed (including XP SP3) and a good anti-malware software as well.
Be sure to download which ever you select, close all programs, disconnect from the Internet and uninstall the A/V currently installed. Then install the program you selected. Allow the software to update.
Let us know if you have any other questions.