I am too on P III 1.0 GHz, 128 MB SDRAM, Windows XP and I am using Kaspersky Personal and Kerio Personal Firewall, I enable real time protection, only when I am online, I have not performed "Full System Scan" for last two months or so, My PC is working fine.
F-Secure, Mcafee Enterprise Edition [90-day Trial Available] are other antivirus worth to Try.